Add smoke tests for UOM conversions and enhance frontend UOM management

- Implemented smoke tests for UOM directionality, ensuring conversions are one-directional and correctly validated.
- Added tests for receiving and selling items in different UOMs, verifying correct quantity handling and error responses.
- Created a UOM conversions panel in the frontend to allow users to manage UOM conversions for items.
- Introduced hooks for allowed UOMs to optimize fetching and caching of UOM data for document line forms.
- Developed utility functions for consistent UOM formatting and conversion handling across the application.

Backend/ERPCore/Controllers/ItemsController.cs

@@ -1,6 +1,7 @@
 using ERPCore.Domain.Enums;
 using ERPCore.Dtos.Common;
 using ERPCore.Dtos.Items;
+using ERPCore.Dtos.Uoms;
 using ERPCore.Services.Interfaces;
 using Microsoft.AspNetCore.Mvc;
 
@@ -11,8 +12,13 @@ namespace ERPCore.Controllers;
 public sealed class ItemsController : ApiControllerBase
 {
     private readonly IItemService _items;
+    private readonly IUomConverter _uomConverter;
 
-    public ItemsController(IItemService items) => _items = items;
+    public ItemsController(IItemService items, IUomConverter uomConverter)
+    {
+        _items = items;
+        _uomConverter = uomConverter;
+    }
 
     /// <summary>List items with optional filters and paging.</summary>
     [HttpGet]
@@ -88,4 +94,15 @@ public sealed class ItemsController : ApiControllerBase
     [ProducesResponseType(StatusCodes.Status404NotFound)]
     public async Task<ActionResult<ItemUomConversionsDto>> UpdateUomConversions(int itemId, [FromBody] UpdateUomConversionsRequest request, CancellationToken ct)
         => Ok(await _items.UpdateUomConversionsAsync(itemId, request, ct));
+
+    /// <summary>
+    /// The UOMs this item may be transacted in — its base UOM plus every UOM it has a
+    /// conversion from, each with the factor to base. Document line forms use this to offer
+    /// only units that will survive posting, instead of the whole global UOM list (FR-MD-02/03).
+    /// </summary>
+    [HttpGet("{itemId:int}/uoms")]
+    [ProducesResponseType(typeof(IReadOnlyList<AllowedUomDto>), StatusCodes.Status200OK)]
+    [ProducesResponseType(StatusCodes.Status404NotFound)]
+    public async Task<ActionResult<IReadOnlyList<AllowedUomDto>>> GetAllowedUoms(int itemId, CancellationToken ct)
+        => Ok(await _uomConverter.GetAllowedUomsAsync(itemId, ct));
 }

Backend/ERPCore/Domain/Entities/BundleSaleLine.cs

@@ -11,9 +11,20 @@ public class BundleSaleLine
     public int ItemId { get; set; }
     public Item? Item { get; set; }
     public string Description { get; set; } = string.Empty;
+    /// <summary>Component quantity, in <see cref="UomId"/> — what the user entered.</summary>
     public decimal Qty { get; set; }
     public int UomId { get; set; }
     public Uom? Uom { get; set; }
+
+    /// <summary>
+    /// <see cref="Qty"/> restated in the item's base UOM, resolved once at save. Posting
+    /// consumes this. <see cref="Qty"/> and <see cref="UnitPrice"/> are always a matching
+    /// pair in <see cref="UomId"/>, so <see cref="LineTotal"/> stays value-correct.
+    /// </summary>
+    public decimal QtyBase { get; set; }
+
+    /// <summary>The factor used to derive <see cref="QtyBase"/>; 1 when the line is in base UOM.</summary>
+    public decimal ConversionFactor { get; set; } = 1m;
     public int WarehouseId { get; set; }
     public Warehouse? Warehouse { get; set; }
     public decimal UnitPrice { get; set; }

Backend/ERPCore/Domain/Entities/GrnLine.cs

@@ -34,8 +34,20 @@ public class GrnLine
     public int? BatchId { get; set; }
     public Batch? Batch { get; set; }
 
+    /// <summary>Quantity received, in <see cref="UomId"/> — what the user entered.</summary>
     public decimal Qty { get; set; }
 
+    /// <summary>
+    /// <see cref="Qty"/> restated in the item's base UOM, resolved once at line creation.
+    /// Confirm reads this snapshot rather than re-converting, so a conversion factor edited
+    /// between create and confirm cannot change what a saved GRN posts, and a later reversal
+    /// reproduces the original layer exactly.
+    /// </summary>
+    public decimal QtyBase { get; set; }
+
+    /// <summary>The factor used to derive <see cref="QtyBase"/>; 1 when the line is in base UOM.</summary>
+    public decimal ConversionFactor { get; set; } = 1m;
+
     /// <summary>Gross unit cost received at (entered, or PO price when omitted).</summary>
     public decimal UnitCost { get; set; }
 

Backend/ERPCore/Domain/Entities/PoLine.cs

@@ -21,8 +21,32 @@ public class PoLine
     public int WarehouseId { get; set; }
     public Warehouse? Warehouse { get; set; }
 
+    /// <summary>Quantity ordered, in <see cref="UomId"/>.</summary>
     public decimal Qty { get; set; }
     public decimal UnitPrice { get; set; }//
     public decimal Tax { get; set; }
+
+    /// <summary>
+    /// <see cref="Qty"/> restated in the item's base UOM, resolved once at line creation.
+    /// This — not <see cref="Qty"/> — is what open-quantity and close checks compare against,
+    /// because a GRN may legitimately receive against this line in a different UOM.
+    /// </summary>
+    public decimal QtyBase { get; set; }
+
+    /// <summary>The factor used to derive <see cref="QtyBase"/>; 1 when the line is in base UOM.</summary>
+    public decimal ConversionFactor { get; set; } = 1m;
+
+    /// <summary>
+    /// Accrues as GRNs confirm (FR-PROC-07), in <see cref="UomId"/>. <b>Denormalized for
+    /// display only</b> — it is derived by dividing <see cref="QtyReceivedBase"/> by the
+    /// factor, so it can drift. Never branch on it; use <see cref="QtyReceivedBase"/>.
+    /// </summary>
     public decimal QtyReceived { get; set; }
+
+    /// <summary>
+    /// Authoritative received-to-date in the item's base UOM. GRN confirm accrues here and
+    /// the PO close condition compares this against <see cref="QtyBase"/>, so receipts in a
+    /// UOM other than the PO's still add up correctly.
+    /// </summary>
+    public decimal QtyReceivedBase { get; set; }
 }

Backend/ERPCore/Domain/Entities/SalesInvoiceLine.cs

@@ -14,10 +14,24 @@ public class SalesInvoiceLine
 
     public string Description { get; set; } = string.Empty;
 
+    /// <summary>Quantity sold, in <see cref="UomId"/> — what the user entered and what prints.</summary>
     public decimal Qty { get; set; }
     public decimal FreeQty { get; set; }
     public int UomId { get; set; }
     public Uom? Uom { get; set; }
+
+    /// <summary>
+    /// <see cref="Qty"/> restated in the item's base UOM, resolved once at save. Posting
+    /// consumes stock against this snapshot — the FIFO engine is base-UOM only, so passing
+    /// the entered quantity would deplete the wrong amount whenever the line is not in base UOM.
+    /// </summary>
+    public decimal QtyBase { get; set; }
+
+    /// <summary><see cref="FreeQty"/> restated in the item's base UOM.</summary>
+    public decimal FreeQtyBase { get; set; }
+
+    /// <summary>The factor used to derive the base quantities; 1 when the line is in base UOM.</summary>
+    public decimal ConversionFactor { get; set; } = 1m;
     public int WarehouseId { get; set; }
     public Warehouse? Warehouse { get; set; }
 

Backend/ERPCore/Domain/Entities/SalesSlipLine.cs

@@ -14,10 +14,24 @@ public class SalesSlipLine
 
     public string Description { get; set; } = string.Empty;
 
+    /// <summary>Quantity sold, in <see cref="UomId"/> — what the user entered and what prints.</summary>
     public decimal Qty { get; set; }
     public decimal FreeQty { get; set; }
     public int UomId { get; set; }
     public Uom? Uom { get; set; }
+
+    /// <summary>
+    /// <see cref="Qty"/> restated in the item's base UOM, resolved once at save. Posting
+    /// consumes stock against this snapshot — the FIFO engine is base-UOM only, so passing
+    /// the entered quantity would deplete the wrong amount whenever the line is not in base UOM.
+    /// </summary>
+    public decimal QtyBase { get; set; }
+
+    /// <summary><see cref="FreeQty"/> restated in the item's base UOM.</summary>
+    public decimal FreeQtyBase { get; set; }
+
+    /// <summary>The factor used to derive the base quantities; 1 when the line is in base UOM.</summary>
+    public decimal ConversionFactor { get; set; } = 1m;
     public int WarehouseId { get; set; }
     public Warehouse? Warehouse { get; set; }
 

Backend/ERPCore/Dtos/Procurement/PurchaseOrderDtos.cs

@@ -5,9 +5,18 @@ namespace ERPCore.Dtos.Procurement;
 
 // Responses (docs/11 §3.3) ------------------------------------------------------
 
+/// <summary>
+/// A PO line. <paramref name="Qty"/> and <paramref name="QtyReceived"/> are in
+/// <paramref name="UomId"/> and are what the user sees; <paramref name="QtyBase"/> and
+/// <paramref name="QtyReceivedBase"/> are the item's base UOM and are what the server
+/// actually enforces — GRN over-receipt and the PO close condition both run on the base
+/// pair, because goods may legitimately be received in a different UOM from the one
+/// ordered. A client showing remaining/outstanding quantity should read the base pair.
+/// </summary>
 public sealed record PoLineDto(
     int PoLineId, int ItemId, int UomId, int WarehouseId,
-    decimal Qty, decimal UnitPrice, decimal Tax, decimal QtyReceived);
+    decimal Qty, decimal UnitPrice, decimal Tax, decimal QtyReceived,
+    decimal QtyBase = 0m, decimal QtyReceivedBase = 0m, decimal ConversionFactor = 1m);
 
 public sealed record PoTotalsDto(decimal SubTotal, decimal Tax, decimal GrandTotal, string Currency);
 

Backend/ERPCore/Dtos/Sales/BundleSaleDtos.cs

@@ -31,9 +31,10 @@ public sealed record BundleSaleTemplateSummaryDto(
     int BundleSaleTemplateId, string TemplateCode, string TemplateName, string? Description,
     EntityStatus Status, int LineCount, DateTime CreatedAt, DateTime? UpdatedAt);
 
+/// <summary>Quantities are in the item's base UOM — see <c>SalesInvoicePostingIssueDto</c>.</summary>
 public sealed record BundleSalePostingIssueDto(
     int BundleSaleLineId, int ItemId, string ItemSku, string ItemName, int WarehouseId,
-    decimal RequestedQty, decimal AvailableQty, decimal ShortQty);
+    decimal RequestedQty, decimal AvailableQty, decimal ShortQty, string BaseUomName = "");
 
 public sealed record BundleSalePostingCheckDto(
     int BundleSaleId, string BundleNo, BundleSaleStatus Status, bool CanPost,

Backend/ERPCore/Dtos/Sales/SalesInvoiceDtos.cs

@@ -24,9 +24,16 @@ public sealed record SalesInvoiceSummaryDto(
     string CustomerSnapshotName, int WarehouseId, SalesInvoiceType InvoiceType,
     SalesInvoiceStatus Status, SalesInvoiceTotalsDto Totals, DateTime CreatedAt);
 
+/// <summary>
+/// A line that cannot be posted for lack of stock. The three quantities are in the item's
+/// <b>base</b> UOM (on-hand only exists in base), which may differ from the UOM shown on the
+/// line — hence <paramref name="BaseUomName"/>: without it a line reading "2 BOX" produces
+/// an unexplained "requested 24, available 10".
+/// </summary>
 public sealed record SalesInvoicePostingIssueDto(
     int SalesInvoiceLineId, int ItemId, string ItemSku, string ItemName, int WarehouseId,
-    decimal RequestedQty, decimal AvailableQty, decimal ShortQty, bool IsFreeIssue);
+    decimal RequestedQty, decimal AvailableQty, decimal ShortQty, bool IsFreeIssue,
+    string BaseUomName = "");
 
 public sealed record SalesInvoicePostingCheckDto(
     int SalesInvoiceId, string InvoiceNo, SalesInvoiceStatus Status, bool CanPost,

Backend/ERPCore/Dtos/Sales/SalesSlipDtos.cs

@@ -23,9 +23,11 @@ public sealed record SalesSlipSummaryDto(
     string CustomerSnapshotName, int WarehouseId, SalesSlipStatus Status,
     SalesSlipTotalsDto Totals, DateTime CreatedAt);
 
+/// <summary>Quantities are in the item's base UOM — see <c>SalesInvoicePostingIssueDto</c>.</summary>
 public sealed record SalesSlipPostingIssueDto(
     int SalesSlipLineId, int ItemId, string ItemSku, string ItemName, int WarehouseId,
-    decimal RequestedQty, decimal AvailableQty, decimal ShortQty, bool IsFreeIssue);
+    decimal RequestedQty, decimal AvailableQty, decimal ShortQty, bool IsFreeIssue,
+    string BaseUomName = "");
 
 public sealed record SalesSlipPostingCheckDto(
     int SalesSlipId, string SlipNo, SalesSlipStatus Status, bool CanPost,

Backend/ERPCore/Dtos/Stock/StockDtos.cs

@@ -2,16 +2,23 @@ using ERPCore.Domain.Enums;
 
 namespace ERPCore.Dtos.Stock;
 
+// Every quantity a stock endpoint returns is in the item's base UOM — stock, layers and the
+// ledger are base-only by construction. The BaseUomId/BaseUomName pair on each of these DTOs
+// exists so a client can *label* those figures; without it every stock screen renders a bare
+// number the user has to guess the unit of. They are never a conversion instruction.
+
 /// <summary>Stock enquiry (docs/11 §5.1). available = onHand − onHold − reserved − inTransit(out).</summary>
 public sealed record StockOnHandDto(
     int ItemId, int WarehouseId, decimal OnHand, decimal Available,
-    decimal OnHold, decimal InTransit, decimal Reserved, DateTime AsOf);
+    decimal OnHold, decimal InTransit, decimal Reserved, DateTime AsOf,
+    int BaseUomId = 0, string BaseUomName = "");
 
 /// <summary>A stock-ledger row (docs/11 §5.2).</summary>
 public sealed record StockLedgerRowDto(
     int LedgerId, int ItemId, int WarehouseId, int? BinId, int? BatchId, int? SerialId,
     Direction Direction, decimal QtyBase, decimal UnitCost, decimal Value, decimal RunningBalance,
-    string SourceDocType, int SourceDocId, int UserId, DateTime CreatedAt);
+    string SourceDocType, int SourceDocId, int UserId, DateTime CreatedAt,
+    int BaseUomId = 0, string BaseUomName = "");
 
 /// <summary>An open FIFO layer in a valuation (docs/11 §5.3).</summary>
 public sealed record StockValuationLayerDto(int LayerId, decimal QtyRemaining, decimal UnitCost, decimal Value, DateTime ReceiptDate);
@@ -19,4 +26,5 @@ public sealed record StockValuationLayerDto(int LayerId, decimal QtyRemaining, d
 /// <summary>Valuation of on-hand stock from open FIFO layers (docs/11 §5.3).</summary>
 public sealed record StockValuationDto(
     int ItemId, int WarehouseId, IReadOnlyList<StockValuationLayerDto> Layers,
-    decimal TotalQty, decimal TotalValue, string Currency, string CostingMethod);
+    decimal TotalQty, decimal TotalValue, string Currency, string CostingMethod,
+    int BaseUomId = 0, string BaseUomName = "");

Backend/ERPCore/Dtos/Uoms/UomDtos.cs

@@ -9,3 +9,12 @@ public sealed class CreateUomRequest
 {
     [Required, StringLength(50)] public string Name { get; set; } = string.Empty;
 }
+
+/// <summary>
+/// A UOM an item may actually be transacted in: its base UOM (<paramref name="Factor"/> 1,
+/// <paramref name="IsBase"/> true) plus every UOM it has a conversion from. Backs both
+/// entry-time validation and <c>GET /items/{itemId}/uoms</c>, so the client can offer only
+/// units that will survive posting instead of the whole global list.
+/// </summary>
+/// <param name="Factor">Multiply a quantity in this UOM by <paramref name="Factor"/> to get base UOM.</param>
+public sealed record AllowedUomDto(int UomId, string Name, decimal Factor, bool IsBase);

Backend/ERPCore/Infra/Persistence/Configurations/BundleSaleLineConfiguration.cs

@@ -12,6 +12,8 @@ public sealed class BundleSaleLineConfiguration : IEntityTypeConfiguration<Bundl
         builder.HasKey(x => x.BundleSaleLineId);
         builder.Property(x => x.Description).IsRequired().HasMaxLength(200);
         builder.Property(x => x.Qty).HasPrecision(18, 4);
+        builder.Property(x => x.QtyBase).HasPrecision(18, 4);
+        builder.Property(x => x.ConversionFactor).HasPrecision(18, 6).HasDefaultValue(1m);
         builder.Property(x => x.UnitPrice).HasPrecision(18, 4);
         builder.Property(x => x.LineTotal).HasPrecision(18, 4);
         builder.Property(x => x.IncludeInBundle).HasDefaultValue(true);

Backend/ERPCore/Infra/Persistence/Configurations/GrnConfiguration.cs

@@ -36,6 +36,8 @@ public sealed class GrnLineConfiguration : IEntityTypeConfiguration<GrnLine>
         builder.HasKey(l => l.GrnLineId);
 
         builder.Property(l => l.Qty).HasPrecision(18, 4);
+        builder.Property(l => l.QtyBase).HasPrecision(18, 4);
+        builder.Property(l => l.ConversionFactor).HasPrecision(18, 6).HasDefaultValue(1m);
         builder.Property(l => l.UnitCost).HasPrecision(18, 6);
         builder.Property(l => l.PoUnitPrice).HasPrecision(18, 6);
         builder.Property(l => l.DiscountPct).HasPrecision(9, 4);

Backend/ERPCore/Infra/Persistence/Configurations/PurchaseOrderConfiguration.cs

@@ -49,6 +49,9 @@ public sealed class PoLineConfiguration : IEntityTypeConfiguration<PoLine>
         builder.HasKey(l => l.PoLineId);
 
         builder.Property(l => l.Qty).HasPrecision(18, 4);
+        builder.Property(l => l.QtyBase).HasPrecision(18, 4);
+        builder.Property(l => l.QtyReceivedBase).HasPrecision(18, 4);
+        builder.Property(l => l.ConversionFactor).HasPrecision(18, 6).HasDefaultValue(1m);
         builder.Property(l => l.UnitPrice).HasPrecision(18, 4);
         builder.Property(l => l.Tax).HasPrecision(9, 4);
         builder.Property(l => l.QtyReceived).HasPrecision(18, 4);

Backend/ERPCore/Infra/Persistence/Configurations/SalesInvoiceConfiguration.cs

@@ -70,6 +70,9 @@ public sealed class SalesInvoiceLineConfiguration : IEntityTypeConfiguration<Sal
 
         builder.Property(x => x.Qty).HasPrecision(18, 4);
         builder.Property(x => x.FreeQty).HasPrecision(18, 4);
+        builder.Property(x => x.QtyBase).HasPrecision(18, 4);
+        builder.Property(x => x.FreeQtyBase).HasPrecision(18, 4);
+        builder.Property(x => x.ConversionFactor).HasPrecision(18, 6).HasDefaultValue(1m);
         builder.Property(x => x.UnitPrice).HasPrecision(18, 4);
         builder.Property(x => x.BaseCost).HasPrecision(18, 4);
         builder.Property(x => x.DiscountPct).HasPrecision(9, 4);

Backend/ERPCore/Infra/Persistence/Configurations/SalesSlipConfiguration.cs

@@ -70,6 +70,9 @@ public sealed class SalesSlipLineConfiguration : IEntityTypeConfiguration<SalesS
 
         builder.Property(x => x.Qty).HasPrecision(18, 4);
         builder.Property(x => x.FreeQty).HasPrecision(18, 4);
+        builder.Property(x => x.QtyBase).HasPrecision(18, 4);
+        builder.Property(x => x.FreeQtyBase).HasPrecision(18, 4);
+        builder.Property(x => x.ConversionFactor).HasPrecision(18, 6).HasDefaultValue(1m);
         builder.Property(x => x.UnitPrice).HasPrecision(18, 4);
         builder.Property(x => x.BaseCost).HasPrecision(18, 4);
         builder.Property(x => x.DiscountPct).HasPrecision(9, 4);

Backend/ERPCore/Infra/Persistence/Configurations/UomConversionConfiguration.cs

@@ -30,5 +30,9 @@ public sealed class UomConversionConfiguration : IEntityTypeConfiguration<UomCon
 
         // One conversion per (item, from, to) triple.
         builder.HasIndex(c => new { c.ItemId, c.FromUomId, c.ToUomId }).IsUnique();
+
+        // The API guards this too, but UomConverter divides unit cost by the factor — a zero
+        // reaching the table from a seeder or direct SQL would be a divide-by-zero at post time.
+        builder.ToTable(t => t.HasCheckConstraint("ck_uom_conversions_factor_positive", "\"Factor\" > 0"));
     }
 }

Backend/ERPCore/Services/BundleSaleService.cs

@@ -215,16 +215,23 @@ public sealed class BundleSaleService : IBundleSaleService
 
             var item = await _items.Query().AsNoTracking().FirstAsync(x => x.ItemId == r.ItemId, ct);
             await _sales.ValidateSalesLineAsync(warehouseId, r.ItemId, r.UomId, lineWarehouseId, r.Qty, 0m, null, ct);
-            var (qtyBase, unitCostBase) = await _uomConverter.ToBaseAsync(item, r.UomId, r.Qty, r.UnitPrice, ct);
+
-            var calc = _sales.ComputeLine(qtyBase, 0m, unitCostBase, SalesDiscountMode.Amount, 0m, 0m, 0m, 0m, false);
+            // Keep the entered UOM on the line and snapshot the base quantity beside it, the
+            // same shape as invoices and slips. Qty and UnitPrice stay a matching pair in the
+            // entered UOM so LineTotal — which Recalculate rolls into the header — is the
+            // value the user priced; only QtyBase crosses into the base-UOM stock engine.
+            var factor = await _uomConverter.ResolveFactorAsync(item, r.UomId, ct);
+            var calc = _sales.ComputeLine(r.Qty, 0m, r.UnitPrice, SalesDiscountMode.Amount, 0m, 0m, 0m, 0m, false);
             lines.Add(new BundleSaleLine
             {
                 ItemId = r.ItemId,
                 Description = item.Name,
-                Qty = qtyBase,
+                Qty = r.Qty,
-                UomId = item.BaseUomId,
+                UomId = r.UomId,
+                QtyBase = UomConverter.ApplyFactor(r.Qty, factor),
+                ConversionFactor = factor,
                 WarehouseId = lineWarehouseId,
-                UnitPrice = unitCostBase,
+                UnitPrice = r.UnitPrice,
                 LineTotal = calc.LineTotal,
                 IncludeInBundle = r.IncludeInBundle,
                 IsComponent = true,

Backend/ERPCore/Services/GrnService.cs

@@ -7,6 +7,7 @@ using ERPCore.Infra.Auth;
 using ERPCore.Infra.UoW;
 using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Interfaces;
+using ERPCore.Services.Stock;
 using ERPCore.System.Errors;
 using Microsoft.EntityFrameworkCore;
 
@@ -138,6 +139,12 @@ public sealed class GrnService : IGrnService
             if (input.BinId is not null && !await _bins.Query().AnyAsync(b => b.BinId == input.BinId && b.WarehouseId == request.WarehouseId, ct))
                 throw new DomainException(ErrorCodes.Validation, $"Bin {input.BinId} is not in warehouse {request.WarehouseId}.", 422);
 
+            // Resolve the base quantity at entry. This rejects a UOM the item cannot be
+            // received in while the GRN is still a draft, and snapshots the factor so confirm
+            // (and any later reversal) reproduces exactly this quantity.
+            var conversionFactor = await _uomConverter.ResolveFactorAsync(item, input.UomId, ct);
+            var qtyBaseEntered = UomConverter.ApplyFactor(input.Qty, conversionFactor);
+
             // Cost: for a PO line, the PO price is used unless an override is entered (then it
             // wins and a variance is recorded against the PO snapshot — docs/02-SECURITY C.3,
             // revised). Direct receipts always use the entered cost.
@@ -150,10 +157,13 @@ public sealed class GrnService : IGrnService
                 if (poLine.ItemId != input.ItemId)
                     throw new DomainException(ErrorCodes.Validation, $"PO line {input.PoLineId} is for a different item.", 422);
 
-                var openQty = poLine.Qty - poLine.QtyReceived;
+                // Compare in base UOM: a GRN may legitimately receive in a different UOM from
-                if (input.Qty > openQty * (1 + OverReceiptTolerance))
+                // the one the PO was raised in (10 BOX ordered, 120 PCS delivered), and
+                // comparing the two raw numbers would reject that valid receipt.
+                var openQtyBase = poLine.QtyBase - poLine.QtyReceivedBase;
+                if (qtyBaseEntered > openQtyBase * (1 + OverReceiptTolerance))
                     throw new DomainException(ErrorCodes.OverReceiptTolerance,
-                        $"Receiving {input.Qty} exceeds the open quantity {openQty} on PO line {input.PoLineId}.", 422);
+                        $"Receiving {input.Qty} ({qtyBaseEntered} base) exceeds the open quantity {openQtyBase} base on PO line {input.PoLineId}.", 422);
 
                 poUnitPrice = poLine.UnitPrice;
                 unitCost = input.UnitCost > 0 ? input.UnitCost : poLine.UnitPrice;
@@ -178,6 +188,8 @@ public sealed class GrnService : IGrnService
                 BinId = input.BinId,
                 Batch = batch, // navigation so EF fixes up BatchId once the batch is inserted
                 Qty = input.Qty,
+                QtyBase = qtyBaseEntered,
+                ConversionFactor = conversionFactor,
                 UnitCost = unitCost,
                 PoUnitPrice = poUnitPrice,
                 DiscountPct = input.DiscountPct,
@@ -234,10 +246,13 @@ public sealed class GrnService : IGrnService
         {
             foreach (var line in grn.Lines.OrderBy(l => l.GrnLineId))
             {
-                var item = await _items.Query().AsNoTracking().FirstAsync(i => i.ItemId == line.ItemId, token);
                 // FIFO layer costs at the after-discount net price; VAT is recoverable and never
-                // enters stock value (docs/10 FR-GRN-06, revised).
+                // enters stock value (docs/10 FR-GRN-06, revised). Quantity and cost come from
-                var (qtyBase, unitCostBase) = await ToBaseAsync(item, line.UomId, line.Qty, line.NetUnitCost, token);
+                // the factor snapshotted at line creation, not a fresh lookup — see GrnLine.QtyBase.
+                var qtyBase = line.QtyBase;
+                var unitCostBase = line.ConversionFactor == 1m
+                    ? line.NetUnitCost
+                    : Math.Round(line.NetUnitCost / line.ConversionFactor, 6, MidpointRounding.AwayFromZero);
 
                 var layer = await _fifo.CreateInboundLayerAsync(
                     line.ItemId, grn.WarehouseId, line.BatchId, null, line.GrnLineId,
@@ -258,7 +273,13 @@ public sealed class GrnService : IGrnService
                 if (line.PoLineId is not null)
                 {
                     var poLine = await _poLines.GetByIdAsync(line.PoLineId.Value, token);
-                    if (poLine is not null) poLine.QtyReceived += line.Qty;
+                    if (poLine is not null)
+                    {
+                        // Accrue in base so receipts in a UOM other than the PO's still add up.
+                        // QtyReceived is kept in the PO's own UOM for display only.
+                        poLine.QtyReceivedBase += qtyBase;
+                        poLine.QtyReceived = UomConverter.FromBase(poLine.QtyReceivedBase, poLine.ConversionFactor);
+                    }
                 }
             }
 
@@ -345,22 +366,14 @@ public sealed class GrnService : IGrnService
         return created; // linked via GrnLine.Batch navigation; FK fixed up on SaveChanges
     }
 
-    /// <summary>
-    /// Delegates to the shared <see cref="IUomConverter"/>. This was a private method here
-    /// until manufacturing needed the same conversion for stage stock inputs; behaviour is
-    /// identical, so receive costing is unchanged.
-    /// </summary>
-    private Task<(decimal QtyBase, decimal UnitCostBase)> ToBaseAsync(
-        Item item, int uomId, decimal qty, decimal unitCostPerUom, CancellationToken ct)
-        => _uomConverter.ToBaseAsync(item, uomId, qty, unitCostPerUom, ct);
-
     private async Task UpdatePoStatusAsync(int? poId, CancellationToken ct)
     {
         if (poId is null) return;
         var po = await _pos.Query().Include(p => p.Lines).FirstOrDefaultAsync(p => p.PoId == poId, ct);
         if (po is null) return;
 
-        po.Status = po.Lines.All(l => l.QtyReceived >= l.Qty)
+        // Base-vs-base: QtyReceived is a denormalized display figure and must not gate closing.
+        po.Status = po.Lines.All(l => l.QtyReceivedBase >= l.QtyBase)
             ? PurchaseOrderStatus.FullyReceived
             : PurchaseOrderStatus.PartiallyReceived;
         po.UpdatedAt = DateTime.UtcNow;

Backend/ERPCore/Services/Interfaces/IUomConverter.cs

@@ -1,4 +1,5 @@
 using ERPCore.Domain.Entities;
+using ERPCore.Dtos.Uoms;
 
 namespace ERPCore.Services.Interfaces;
 
@@ -32,4 +33,29 @@ public interface IUomConverter
     /// consumes, not from the document).
     /// </summary>
     Task<decimal> ToBaseQtyAsync(Item item, int uomId, decimal qty, CancellationToken ct = default);
+
+    /// <summary>
+    /// The single factor lookup every other method is built on: <c>1</c> when
+    /// <paramref name="uomId"/> is already the base UOM, otherwise the item's conversion
+    /// factor from that UOM to base. Throws 422 when none is defined.
+    /// </summary>
+    /// <remarks>
+    /// Callers that persist a line should store this alongside the quantity so posting
+    /// reads a snapshot instead of re-resolving — a factor edited between save and post
+    /// must never change what an already-saved document posts.
+    /// </remarks>
+    Task<decimal> ResolveFactorAsync(Item item, int uomId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Every UOM <paramref name="itemId"/> may be transacted in — base UOM first, then each
+    /// conversion source, ordered by name.
+    /// </summary>
+    Task<IReadOnlyList<AllowedUomDto>> GetAllowedUomsAsync(int itemId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Entry-time guard: throws 422 naming the allowed units when <paramref name="uomId"/>
+    /// is neither the item's base UOM nor a UOM it has a conversion from. Call this when a
+    /// document line is created so the user is told at entry, not by a cryptic failure at post.
+    /// </summary>
+    Task ValidateUomAsync(Item item, int uomId, CancellationToken ct = default);
 }

Backend/ERPCore/Services/ItemService.cs

@@ -148,6 +148,15 @@ public sealed class ItemService : IItemService
             request.CategoryId, request.SubCategoryId, request.BrandId,
             request.BaseUomId, request.DefaultVendorId, ct);
 
+        // Conversions are stored as <other> → base. Repointing the base UOM would leave every
+        // existing row aimed at a UOM that is no longer the base: invisible to IUomConverter,
+        // filtered out of the allowed-UOM list, and unfixable through the conversions editor
+        // (which would 422 on re-save). Make the user clear them deliberately instead.
+        if (request.BaseUomId != item.BaseUomId && item.UomConversions.Count > 0)
+            throw new DomainException(ErrorCodes.Validation,
+                $"Item {item.Sku} has {item.UomConversions.Count} UOM conversion(s) defined against base UOM {item.BaseUomId}. " +
+                "Remove them before changing the base UOM, then re-enter them against the new base.", 422);
+
         item.Sku = request.Sku.Trim();
         item.Name = request.Name.Trim();
         item.Description = request.Description;
@@ -240,6 +249,26 @@ public sealed class ItemService : IItemService
             if (!await _uoms.Query().AnyAsync(u => u.UomId == uomId, ct))
                 throw new DomainException(ErrorCodes.Validation, $"UOM {uomId} does not exist.", 422);
 
+        // Conversions are one-directional: always <other> → base. IUomConverter looks up
+        // exactly that shape and never inverts a factor, so a row saved the other way round
+        // would persist happily, render in the UI, and then be invisible at post time. Reject
+        // it here instead of letting it fail later as an unexplained 422.
+        foreach (var c in request.Conversions)
+        {
+            if (c.ToUom != item.BaseUomId)
+                throw new DomainException(ErrorCodes.Validation,
+                    $"Conversion {c.FromUom} → {c.ToUom} is invalid: conversions must convert to the item's base UOM ({item.BaseUomId}).", 422);
+            if (c.FromUom == c.ToUom)
+                throw new DomainException(ErrorCodes.Validation,
+                    $"Conversion {c.FromUom} → {c.ToUom} is invalid: a UOM cannot convert to itself.", 422);
+            if (c.FromUom == item.BaseUomId)
+                throw new DomainException(ErrorCodes.Validation,
+                    "The base UOM converts to itself implicitly (factor 1) and must not be listed.", 422);
+            if (c.Factor <= 0m)
+                throw new DomainException(ErrorCodes.Validation,
+                    $"Conversion {c.FromUom} → {c.ToUom} must have a factor greater than zero.", 422);
+        }
+
         foreach (var stale in item.UomConversions.Where(c => request.Conversions.All(r => r.FromUom != c.FromUomId || r.ToUom != c.ToUomId)).ToList())
             item.UomConversions.Remove(stale);
         foreach (var input in request.Conversions)

Backend/ERPCore/Services/Production/ProductionTemplateService.cs

@@ -27,6 +27,7 @@ public sealed class ProductionTemplateService : IProductionTemplateService
     private readonly IRepository<Item> _items;
     private readonly IRepository<Uom> _uoms;
     private readonly IRepository<ProductionRun> _runs;
+    private readonly IUomConverter _uomConverter;
     private readonly IUnitOfWork _uow;
     private readonly ICurrentUser _currentUser;
 
@@ -34,8 +35,9 @@ public sealed class ProductionTemplateService : IProductionTemplateService
         IRepository<ProductionTemplate> templates, IRepository<TemplateStage> stages,
         IRepository<StageInput> inputs, IRepository<StageOutput> outputs, IRepository<StageEdge> edges,
         IRepository<Item> items, IRepository<Uom> uoms, IRepository<ProductionRun> runs,
-        IUnitOfWork uow, ICurrentUser currentUser)
+        IUomConverter uomConverter, IUnitOfWork uow, ICurrentUser currentUser)
     {
+        _uomConverter = uomConverter;
         _templates = templates;
         _stages = stages;
         _inputs = inputs;
@@ -291,6 +293,20 @@ public sealed class ProductionTemplateService : IProductionTemplateService
             throw new DomainException(ErrorCodes.Validation,
                 $"UOM(s) {string.Join(", ", missingUoms)} do not exist.", 422);
 
+        // Existence is not enough: a stage line's UOM must be one its own item can convert to
+        // base, or the run would fail with a 422 at stage start — long after the template was
+        // authored. Check the (item, uom) pairing here, while the template is being saved.
+        var itemUomPairs = request.Stages
+            .SelectMany(s => s.Inputs.Select(i => (i.ItemId, i.UomId)).Concat(s.Outputs.Select(o => (o.ItemId, o.UomId))))
+            .Distinct().ToList();
+
+        foreach (var (itemId, uomId) in itemUomPairs)
+        {
+            var item = await _items.Query().AsNoTracking().FirstOrDefaultAsync(i => i.ItemId == itemId, ct);
+            if (item is not null)
+                await _uomConverter.ValidateUomAsync(item, uomId, ct);
+        }
+
         // Annotations go into jsonb unvalidated by anything else, so pin the one field the
         // client renders off. Unknown kinds would round-trip fine but draw nothing.
         var badKinds = request.Annotations

Backend/ERPCore/Services/PurchaseOrderService.cs

@@ -8,6 +8,7 @@ using ERPCore.Infra.Auth;
 using ERPCore.Infra.UoW;
 using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Interfaces;
+using ERPCore.Services.Stock;
 using ERPCore.System.Errors;
 using Microsoft.EntityFrameworkCore;
 
@@ -28,6 +29,7 @@ public sealed class PurchaseOrderService : IPurchaseOrderService
     private readonly IRepository<Item> _items;
     private readonly IRepository<Uom> _uoms;
     private readonly IRepository<Warehouse> _warehouses;
+    private readonly IUomConverter _uomConverter;
     private readonly INumberSequenceService _numbers;
     private readonly ICurrentUser _currentUser;
     private readonly IUnitOfWork _uow;
@@ -35,8 +37,9 @@ public sealed class PurchaseOrderService : IPurchaseOrderService
     public PurchaseOrderService(
         IRepository<PurchaseOrder> pos, IRepository<Vendor> vendors, IRepository<Requisition> requisitions,
         IRepository<Item> items, IRepository<Uom> uoms, IRepository<Warehouse> warehouses,
-        INumberSequenceService numbers, ICurrentUser currentUser, IUnitOfWork uow)
+        IUomConverter uomConverter, INumberSequenceService numbers, ICurrentUser currentUser, IUnitOfWork uow)
     {
+        _uomConverter = uomConverter;
         _pos = pos;
         _vendors = vendors;
         _requisitions = requisitions;
@@ -83,6 +86,7 @@ public sealed class PurchaseOrderService : IPurchaseOrderService
     {
         await ValidateReferencesAsync(request.VendorId, request.RequisitionId, request.Lines, ct);
         var actor = _currentUser.AuditUserId;
+        var lines = await ToLinesAsync(request.Lines, ct);
 
         var po = await _uow.ExecuteInTransactionAsync(async token =>
         {
@@ -98,7 +102,7 @@ public sealed class PurchaseOrderService : IPurchaseOrderService
                 Status = request.SaveAsDraft ? PurchaseOrderStatus.Draft : PurchaseOrderStatus.Approved,
                 CreatedBy = actor,
                 CreatedAt = DateTime.UtcNow,
-                Lines = request.Lines.Select(ToLine).ToList()
+                Lines = lines
             };
             await _pos.AddAsync(entity, token);
             return entity;
@@ -129,8 +133,8 @@ public sealed class PurchaseOrderService : IPurchaseOrderService
 
         // Full line replacement (Phase 1: no receipts yet, so qtyReceived is 0 on every line).
         po.Lines.Clear();
-        foreach (var input in request.Lines)
+        foreach (var line in await ToLinesAsync(request.Lines, ct))
-            po.Lines.Add(ToLine(input));
+            po.Lines.Add(line);
 
         try
         {
@@ -219,16 +223,34 @@ public sealed class PurchaseOrderService : IPurchaseOrderService
     // Supersedes Phase-1 Option B "freely editable while open" — see docs/10 FR-PROC-05.
     private static bool IsEditable(PurchaseOrderStatus status) => status is PurchaseOrderStatus.Draft;
 
-    private static PoLine ToLine(CreatePoLineInput l) => new()
+    /// <summary>
+    /// Materialises request lines, resolving each one's base quantity up front. Resolving here
+    /// rejects a UOM the item cannot be ordered in at entry time, and gives GRN receipt matching
+    /// a stable base figure to compare against regardless of the UOM the goods arrive in.
+    /// </summary>
+    private async Task<List<PoLine>> ToLinesAsync(IReadOnlyCollection<CreatePoLineInput> inputs, CancellationToken ct)
     {
-        ItemId = l.ItemId,
+        var lines = new List<PoLine>(inputs.Count);
-        UomId = l.UomId,
+        foreach (var l in inputs)
-        WarehouseId = l.WarehouseId,
+        {
-        Qty = l.Qty,
+            var item = await _items.Query().AsNoTracking().FirstAsync(i => i.ItemId == l.ItemId, ct);
-        UnitPrice = l.UnitPrice,
+            var factor = await _uomConverter.ResolveFactorAsync(item, l.UomId, ct);
-        Tax = l.Tax,
+            lines.Add(new PoLine
-        QtyReceived = 0
+            {
-    };
+                ItemId = l.ItemId,
+                UomId = l.UomId,
+                WarehouseId = l.WarehouseId,
+                Qty = l.Qty,
+                QtyBase = UomConverter.ApplyFactor(l.Qty, factor),
+                ConversionFactor = factor,
+                UnitPrice = l.UnitPrice,
+                Tax = l.Tax,
+                QtyReceived = 0,
+                QtyReceivedBase = 0
+            });
+        }
+        return lines;
+    }
 
     private static PoTotalsDto ComputeTotals(IEnumerable<PoLine> lines)
     {
@@ -276,5 +298,6 @@ public sealed class PurchaseOrderService : IPurchaseOrderService
         p.PoId, p.DocNo, p.VendorId, p.RequisitionId, p.Status, p.ApprovalRequired,
         p.CreatedBy, p.CreatedAt, p.UpdatedAt, ComputeTotals(p.Lines),
         p.Lines.OrderBy(l => l.PoLineId).Select(l => new PoLineDto(
-            l.PoLineId, l.ItemId, l.UomId, l.WarehouseId, l.Qty, l.UnitPrice, l.Tax, l.QtyReceived)).ToList());
+            l.PoLineId, l.ItemId, l.UomId, l.WarehouseId, l.Qty, l.UnitPrice, l.Tax, l.QtyReceived,
+            l.QtyBase, l.QtyReceivedBase, l.ConversionFactor)).ToList());
 }

Backend/ERPCore/Services/SalesInvoiceService.cs

@@ -25,6 +25,7 @@ public sealed class SalesInvoiceService : ISalesInvoiceService
     private readonly ISalesPostingService _posting;
     private readonly ISalesMappingService _mapping;
     private readonly ISalesDocumentWorkflowService _workflow;
+    private readonly IUomConverter _uomConverter;
     private readonly ICurrentUser _currentUser;
     private readonly INumberSequenceService _numbers;
     private readonly IUnitOfWork _uow;
@@ -32,9 +33,10 @@ public sealed class SalesInvoiceService : ISalesInvoiceService
     public SalesInvoiceService(
         IRepository<SalesInvoice> invoices, IRepository<Customer> customers, IRepository<Item> items,
         IRepository<Uom> uoms, IRepository<Warehouse> warehouses, ISalesDomainService sales, ISalesPostingService posting, ISalesMappingService mapping,
-        ISalesDocumentWorkflowService workflow,
+        ISalesDocumentWorkflowService workflow, IUomConverter uomConverter,
         ICurrentUser currentUser, INumberSequenceService numbers, IUnitOfWork uow)
     {
+        _uomConverter = uomConverter;
         _invoices = invoices;
         _customers = customers;
         _items = items;
@@ -152,6 +154,12 @@ public sealed class SalesInvoiceService : ISalesInvoiceService
 
             var calc = _sales.ComputeLine(r.Qty, r.FreeQty, unitPrice, r.DiscountMode, r.DiscountPct, r.DiscountValue, r.DiscountAmount, r.TaxPct, r.IsFreeIssue);
 
+            // Resolve the base quantity now and store it on the line. This doubles as the
+            // entry-time UOM check (an item that cannot be sold in this UOM throws 422 here,
+            // while the user is still editing) and as the snapshot posting consumes — a
+            // conversion factor edited later must not change what this document posts.
+            var factor = await _uomConverter.ResolveFactorAsync(item, r.UomId, ct);
+
             lines.Add(new SalesInvoiceLine
             {
                 ItemId = r.ItemId,
@@ -159,6 +167,9 @@ public sealed class SalesInvoiceService : ISalesInvoiceService
                 Qty = r.Qty,
                 FreeQty = r.FreeQty,
                 UomId = r.UomId,
+                QtyBase = UomConverter.ApplyFactor(r.Qty, factor),
+                FreeQtyBase = UomConverter.ApplyFactor(r.FreeQty, factor),
+                ConversionFactor = factor,
                 WarehouseId = r.WarehouseId,
                 UnitPrice = unitPrice,
                 BaseCost = unitPrice,

Backend/ERPCore/Services/SalesPostingService.cs

@@ -20,10 +20,11 @@ public sealed class SalesPostingService : ISalesPostingService
     private readonly IRepository<Item> _items;
     private readonly IFifoCostingService _fifo;
     private readonly ISalesDomainService _sales;
-    private readonly IUomConverter _uomConverter;
     private readonly ICurrentUser _currentUser;
     private readonly IUnitOfWork _uow;
 
+    // No IUomConverter here by design: every line arrives with its base quantity already
+    // snapshotted by the service that saved it, so posting has nothing left to convert.
     public SalesPostingService(
         IRepository<SalesInvoice> invoices,
         IRepository<SalesSlip> slips,
@@ -31,7 +32,6 @@ public sealed class SalesPostingService : ISalesPostingService
         IRepository<Item> items,
         IFifoCostingService fifo,
         ISalesDomainService sales,
-        IUomConverter uomConverter,
         ICurrentUser currentUser,
         IUnitOfWork uow)
     {
@@ -41,7 +41,6 @@ public sealed class SalesPostingService : ISalesPostingService
         _items = items;
         _fifo = fifo;
         _sales = sales;
-        _uomConverter = uomConverter;
         _currentUser = currentUser;
         _uow = uow;
     }
@@ -60,18 +59,20 @@ public sealed class SalesPostingService : ISalesPostingService
         {
             if (!await _sales.IsStockedItemAsync(line.ItemId, ct))
                 continue;
-            var requestedQty = line.Qty + line.FreeQty;
+            // Compare base against base: on-hand is base UOM, so the entered quantity would
+            // under-report the requirement on any line not in the item's base UOM.
+            var requestedQty = line.QtyBase + line.FreeQtyBase;
             var available = await _fifo.GetOnHandAsync(line.ItemId, line.WarehouseId, ct);
             if (available >= requestedQty) continue;
 
             var item = await _items.Query().AsNoTracking()
                 .Where(x => x.ItemId == line.ItemId)
-                .Select(x => new { x.Sku, x.Name })
+                .Select(x => new { x.Sku, x.Name, BaseUomName = x.BaseUom!.Name })
                 .FirstAsync(ct);
 
             issues.Add(new SalesInvoicePostingIssueDto(
                 line.SalesInvoiceLineId, line.ItemId, item.Sku, item.Name, line.WarehouseId,
-                requestedQty, available, requestedQty - available, line.IsFreeIssue));
+                requestedQty, available, requestedQty - available, line.IsFreeIssue, item.BaseUomName));
         }
 
         return new SalesInvoicePostingCheckDto(invoice.SalesInvoiceId, invoice.InvoiceNo, invoice.Status, issues.Count == 0, issues);
@@ -91,18 +92,19 @@ public sealed class SalesPostingService : ISalesPostingService
         {
             if (!await _sales.IsStockedItemAsync(line.ItemId, ct))
                 continue;
-            var requestedQty = line.Qty + line.FreeQty;
+            // Base against base — see the matching comment in CheckInvoiceAsync.
+            var requestedQty = line.QtyBase + line.FreeQtyBase;
             var available = await _fifo.GetOnHandAsync(line.ItemId, line.WarehouseId, ct);
             if (available >= requestedQty) continue;
 
             var item = await _items.Query().AsNoTracking()
                 .Where(x => x.ItemId == line.ItemId)
-                .Select(x => new { x.Sku, x.Name })
+                .Select(x => new { x.Sku, x.Name, BaseUomName = x.BaseUom!.Name })
                 .FirstAsync(ct);
 
             issues.Add(new SalesSlipPostingIssueDto(
                 line.SalesSlipLineId, line.ItemId, item.Sku, item.Name, line.WarehouseId,
-                requestedQty, available, requestedQty - available, line.IsFreeIssue));
+                requestedQty, available, requestedQty - available, line.IsFreeIssue, item.BaseUomName));
         }
 
         return new SalesSlipPostingCheckDto(slip.SalesSlipId, slip.SlipNo, slip.Status, issues.Count == 0, issues);
@@ -124,12 +126,15 @@ public sealed class SalesPostingService : ISalesPostingService
                 continue;
 
             var item = await _items.Query().AsNoTracking()
-                .FirstAsync(x => x.ItemId == line.ItemId, ct);
+                .Where(x => x.ItemId == line.ItemId)
+                .Select(x => new { x.Sku, x.Name, BaseUomName = x.BaseUom!.Name })
+                .FirstAsync(ct);
             var available = await _fifo.GetOnHandAsync(line.ItemId, line.WarehouseId, ct);
-            if (available >= line.Qty) continue;
+            if (available >= line.QtyBase) continue;
 
             issues.Add(new BundleSalePostingIssueDto(
-                line.BundleSaleLineId, line.ItemId, item.Sku, item.Name, line.WarehouseId, line.Qty, available, line.Qty - available));
+                line.BundleSaleLineId, line.ItemId, item.Sku, item.Name, line.WarehouseId,
+                line.QtyBase, available, line.QtyBase - available, item.BaseUomName));
         }
 
         return new BundleSalePostingCheckDto(bundle.BundleSaleId, bundle.BundleNo, bundle.Status, issues.Count == 0, issues);
@@ -141,7 +146,7 @@ public sealed class SalesPostingService : ISalesPostingService
             notFoundMessage: $"Sales invoice {salesInvoiceId} was not found.",
             statusSelector: x => x.Status,
             ensureDraftMessage: x => $"Sales invoice {x.SalesInvoiceId} is {x.Status} and cannot be posted.",
-            getLines: x => x.Lines.Select(l => new PostingLine(l.ItemId, l.WarehouseId, l.UomId, l.Qty + l.FreeQty, l.Qty, l.FreeQty)),
+            getLines: x => x.Lines.Select(l => new PostingLine(l.ItemId, l.WarehouseId, l.QtyBase + l.FreeQtyBase, l.QtyBase, l.FreeQtyBase)),
             setPosted: x => x.Status = SalesInvoiceStatus.Posted,
             setUpdated: x => x.UpdatedAt = DateTime.UtcNow,
             sourceDocType: DocumentTypes.SalesInvoice,
@@ -154,7 +159,7 @@ public sealed class SalesPostingService : ISalesPostingService
             notFoundMessage: $"Sales slip {salesSlipId} was not found.",
             statusSelector: x => x.Status,
             ensureDraftMessage: x => $"Sales slip {x.SalesSlipId} is {x.Status} and cannot be posted.",
-            getLines: x => x.Lines.Select(l => new PostingLine(l.ItemId, l.WarehouseId, l.UomId, l.Qty + l.FreeQty, l.Qty, l.FreeQty)),
+            getLines: x => x.Lines.Select(l => new PostingLine(l.ItemId, l.WarehouseId, l.QtyBase + l.FreeQtyBase, l.QtyBase, l.FreeQtyBase)),
             setPosted: x => x.Status = SalesSlipStatus.Posted,
             setUpdated: x => x.UpdatedAt = DateTime.UtcNow,
             sourceDocType: DocumentTypes.SalesSlip,
@@ -167,9 +172,7 @@ public sealed class SalesPostingService : ISalesPostingService
             notFoundMessage: $"Bundle sale {bundleSaleId} was not found.",
             statusSelector: x => x.Status,
             ensureDraftMessage: x => $"Bundle sale {x.BundleSaleId} is {x.Status} and cannot be posted.",
-            // Bundle lines are normalized to base UOM on save, so posting should consume the
+            getLines: x => x.Lines.Where(l => l.IncludeInBundle).Select(l => new PostingLine(l.ItemId, l.WarehouseId, l.QtyBase, l.QtyBase, 0m)),
-            // stored base quantity directly instead of converting again.
-            getLines: x => x.Lines.Where(l => l.IncludeInBundle).Select(l => new PostingLine(l.ItemId, l.WarehouseId, l.UomId, l.Qty, l.Qty, 0m)),
             setPosted: x => x.Status = BundleSaleStatus.Posted,
             setUpdated: x => x.UpdatedAt = DateTime.UtcNow,
             sourceDocType: DocumentTypes.BundleSale,
@@ -214,5 +217,11 @@ public sealed class SalesPostingService : ISalesPostingService
         }, ct);
     }
 
-    private sealed record PostingLine(int ItemId, int WarehouseId, int UomId, decimal Qty, decimal PaidQty, decimal FreeQty);
+    /// <summary>
+    /// A line reduced to what posting needs. Every quantity here is in the item's <b>base</b>
+    /// UOM, taken from the snapshot the document service resolved at save — the FIFO engine
+    /// accepts nothing else, and re-resolving at post time would let a factor edited in the
+    /// meantime change what a saved document consumes.
+    /// </summary>
+    private sealed record PostingLine(int ItemId, int WarehouseId, decimal Qty, decimal PaidQty, decimal FreeQty);
 }

Backend/ERPCore/Services/SalesSlipService.cs

@@ -26,6 +26,7 @@ public sealed class SalesSlipService : ISalesSlipService
     private readonly ISalesPostingService _posting;
     private readonly ISalesMappingService _mapping;
     private readonly ISalesDocumentWorkflowService _workflow;
+    private readonly IUomConverter _uomConverter;
     private readonly ICurrentUser _currentUser;
     private readonly INumberSequenceService _numbers;
     private readonly IUnitOfWork _uow;
@@ -33,9 +34,10 @@ public sealed class SalesSlipService : ISalesSlipService
     public SalesSlipService(
         IRepository<SalesSlip> slips, IRepository<Customer> customers, IRepository<Item> items,
         IRepository<Uom> uoms, IRepository<Warehouse> warehouses, IRepository<User> users, ISalesDomainService sales, ISalesPostingService posting, ISalesMappingService mapping,
-        ISalesDocumentWorkflowService workflow,
+        ISalesDocumentWorkflowService workflow, IUomConverter uomConverter,
         ICurrentUser currentUser, INumberSequenceService numbers, IUnitOfWork uow)
     {
+        _uomConverter = uomConverter;
         _slips = slips;
         _customers = customers;
         _items = items;
@@ -176,6 +178,10 @@ public sealed class SalesSlipService : ISalesSlipService
 
             var calc = _sales.ComputeLine(r.Qty, r.FreeQty, unitPrice, r.DiscountMode, r.DiscountPct, r.DiscountValue, r.DiscountAmount, r.TaxPct, r.IsFreeIssue);
 
+            // Resolve the base quantity now and store it on the line — see the matching
+            // comment in SalesInvoiceService.BuildLinesAsync.
+            var factor = await _uomConverter.ResolveFactorAsync(item, r.UomId, ct);
+
             lines.Add(new SalesSlipLine
             {
                 ItemId = r.ItemId,
@@ -183,6 +189,9 @@ public sealed class SalesSlipService : ISalesSlipService
                 Qty = r.Qty,
                 FreeQty = r.FreeQty,
                 UomId = r.UomId,
+                QtyBase = UomConverter.ApplyFactor(r.Qty, factor),
+                FreeQtyBase = UomConverter.ApplyFactor(r.FreeQty, factor),
+                ConversionFactor = factor,
                 WarehouseId = r.WarehouseId,
                 UnitPrice = unitPrice,
                 BaseCost = unitPrice,

Backend/ERPCore/Services/Stock/StockService.cs

@@ -14,17 +14,32 @@ public sealed class StockService : IStockService
     private readonly IRepository<StockLayer> _layers;
     private readonly IRepository<StockLedger> _ledger;
     private readonly IRepository<StockTransferLine> _transferLines;
+    private readonly IRepository<Item> _items;
 
     public StockService(
         IFifoCostingService fifo, IRepository<StockLayer> layers,
-        IRepository<StockLedger> ledger, IRepository<StockTransferLine> transferLines)
+        IRepository<StockLedger> ledger, IRepository<StockTransferLine> transferLines,
+        IRepository<Item> items)
     {
         _fifo = fifo;
         _layers = layers;
         _ledger = ledger;
         _transferLines = transferLines;
+        _items = items;
     }
 
+    /// <summary>
+    /// Base UOM (id + name) for a set of items, as one query. Stock reads are already
+    /// set-based to avoid N+1; this keeps the UOM label on the same footing.
+    /// </summary>
+    private async Task<Dictionary<int, (int Id, string Name)>> BaseUomsAsync(
+        IReadOnlyCollection<int> itemIds, CancellationToken ct)
+        => (await _items.Query().AsNoTracking()
+                .Where(i => itemIds.Contains(i.ItemId))
+                .Select(i => new { i.ItemId, i.BaseUomId, Name = i.BaseUom!.Name })
+                .ToListAsync(ct))
+            .ToDictionary(x => x.ItemId, x => (x.BaseUomId, x.Name));
+
     public async Task<StockOnHandDto> GetOnHandAsync(int itemId, int warehouseId, CancellationToken ct = default)
     {
         var onHand = await _fifo.GetOnHandAsync(itemId, warehouseId, ct);
@@ -47,7 +62,10 @@ public sealed class StockService : IStockService
         const decimal reserved = 0m;
         var available = onHand - onHold - reserved;
 
-        return new StockOnHandDto(itemId, warehouseId, onHand, available, onHold, inTransit, reserved, DateTime.UtcNow);
+        var uom = (await BaseUomsAsync([itemId], ct)).GetValueOrDefault(itemId);
+        return new StockOnHandDto(
+            itemId, warehouseId, onHand, available, onHold, inTransit, reserved, DateTime.UtcNow,
+            uom.Id, uom.Name ?? string.Empty);
     }
 
     /// <summary>
@@ -100,16 +118,20 @@ public sealed class StockService : IStockService
             .ToListAsync(ct))
             .ToDictionary(x => (x.ItemId, x.WarehouseId), x => x.Qty);
 
+        var baseUoms = await BaseUomsAsync(itemIds, ct);
+
         var asOf = DateTime.UtcNow;
         var rows = page.Select(p =>
         {
             var key = (p.ItemId, p.WarehouseId);
             var hold = onHold.GetValueOrDefault(key);
             var transit = inTransit.GetValueOrDefault(key);
+            var uom = baseUoms.GetValueOrDefault(p.ItemId);
             const decimal reserved = 0m;
             // Same formula as GetOnHandAsync: in-transit is reported, not re-subtracted.
             return new StockOnHandDto(
-                p.ItemId, p.WarehouseId, p.OnHand, p.OnHand - hold - reserved, hold, transit, reserved, asOf);
+                p.ItemId, p.WarehouseId, p.OnHand, p.OnHand - hold - reserved, hold, transit, reserved, asOf,
+                uom.Id, uom.Name ?? string.Empty);
         }).ToList();
 
         return PagedResponse<StockOnHandDto>.Create(rows, query.Page, query.PageSize, total);
@@ -139,9 +161,20 @@ public sealed class StockService : IStockService
                 l.SourceDocType, l.SourceDocId, l.UserId, l.CreatedAt))
             .ToListAsync(ct);
 
+        var baseUoms = await BaseUomsAsync(rows.Select(r => r.ItemId).Distinct().ToList(), ct);
+        rows = rows.Select(r =>
+        {
+            var uom = baseUoms.GetValueOrDefault(r.ItemId);
+            return r with { BaseUomId = uom.Id, BaseUomName = uom.Name ?? string.Empty };
+        }).ToList();
+
         return PagedResponse<StockLedgerRowDto>.Create(rows, query.Page, query.PageSize, total);
     }
 
-    public Task<StockValuationDto> GetValuationAsync(int itemId, int warehouseId, CancellationToken ct = default)
+    public async Task<StockValuationDto> GetValuationAsync(int itemId, int warehouseId, CancellationToken ct = default)
-        => _fifo.GetValuationAsync(itemId, warehouseId, ct);
+    {
+        var valuation = await _fifo.GetValuationAsync(itemId, warehouseId, ct);
+        var uom = (await BaseUomsAsync([itemId], ct)).GetValueOrDefault(itemId);
+        return valuation with { BaseUomId = uom.Id, BaseUomName = uom.Name ?? string.Empty };
+    }
 }

Backend/ERPCore/Services/Stock/UomConverter.cs

@@ -1,4 +1,5 @@
 using ERPCore.Domain.Entities;
+using ERPCore.Dtos.Uoms;
 using ERPCore.Repositories.Interfaces;
 using ERPCore.Services.Interfaces;
 using ERPCore.System.Errors;
@@ -11,28 +12,128 @@ namespace ERPCore.Services.Stock;
 /// unchanged from the <c>GrnService.ToBaseAsync</c> it was extracted from, so the GRN
 /// receive path keeps costing exactly as before.
 /// </summary>
+/// <remarks>
+/// Conversions are one-directional by design: a row always reads
+/// <c>FromUom → ToUom = item.BaseUomId</c>, enforced on write by
+/// <c>ItemService.UpdateUomConversionsAsync</c>. Nothing here inverts a factor, so a row
+/// stored in the opposite direction would be invisible to every caller — which is exactly
+/// why the write side rejects it rather than this side guessing.
+/// </remarks>
 public sealed class UomConverter : IUomConverter
 {
-    private readonly IRepository<UomConversion> _conversions;
+    /// <summary>Quantity columns are <c>(18,4)</c> across the model.</summary>
+    private const int QtyScale = 4;
 
-    public UomConverter(IRepository<UomConversion> conversions) => _conversions = conversions;
+    /// <summary>Unit-cost columns and <c>uom_conversions.Factor</c> are <c>(18,6)</c>.</summary>
+    private const int CostScale = 6;
+
+    private readonly IRepository<UomConversion> _conversions;
+    private readonly IRepository<Item> _items;
+    private readonly IRepository<Uom> _uoms;
+
+    public UomConverter(IRepository<UomConversion> conversions, IRepository<Item> items, IRepository<Uom> uoms)
+    {
+        _conversions = conversions;
+        _items = items;
+        _uoms = uoms;
+    }
 
     public async Task<(decimal QtyBase, decimal UnitCostBase)> ToBaseAsync(
         Item item, int uomId, decimal qty, decimal unitCostPerUom, CancellationToken ct = default)
     {
-        if (uomId == item.BaseUomId)
+        var factor = await ResolveFactorAsync(item, uomId, ct);
+        if (factor == 1m)
             return (qty, unitCostPerUom);
 
-        var conv = await _conversions.Query().AsNoTracking()
-            .FirstOrDefaultAsync(c => c.ItemId == item.ItemId && c.FromUomId == uomId && c.ToUomId == item.BaseUomId, ct)
-            ?? throw new DomainException(ErrorCodes.Validation,
-                $"No UOM conversion from {uomId} to base UOM {item.BaseUomId} for item {item.ItemId}.", 422);
-
         // Quantity scales up by the factor, so the per-unit cost scales down by it —
-        // total value is preserved.
+        // total value is preserved. Round to each column's own scale here rather than
-        return (qty * conv.Factor, unitCostPerUom / conv.Factor);
+        // letting the provider truncate on write, so what posts is what was computed.
+        return (
+            Math.Round(qty * factor, QtyScale, MidpointRounding.AwayFromZero),
+            Math.Round(unitCostPerUom / factor, CostScale, MidpointRounding.AwayFromZero));
     }
 
     public async Task<decimal> ToBaseQtyAsync(Item item, int uomId, decimal qty, CancellationToken ct = default)
         => (await ToBaseAsync(item, uomId, qty, 0m, ct)).QtyBase;
+
+    public async Task<decimal> ResolveFactorAsync(Item item, int uomId, CancellationToken ct = default)
+    {
+        if (uomId == item.BaseUomId)
+            return 1m;
+
+        var conv = await _conversions.Query().AsNoTracking()
+            .FirstOrDefaultAsync(c => c.ItemId == item.ItemId && c.FromUomId == uomId && c.ToUomId == item.BaseUomId, ct);
+
+        if (conv is null)
+            throw await NoConversionAsync(item, uomId, ct);
+
+        return conv.Factor;
+    }
+
+    public async Task<IReadOnlyList<AllowedUomDto>> GetAllowedUomsAsync(int itemId, CancellationToken ct = default)
+    {
+        var item = await _items.Query().AsNoTracking()
+            .Where(i => i.ItemId == itemId)
+            .Select(i => new { i.ItemId, i.BaseUomId, BaseUomName = i.BaseUom!.Name })
+            .FirstOrDefaultAsync(ct)
+            ?? throw new NotFoundException($"Item {itemId} was not found.");
+
+        var converted = await _conversions.Query().AsNoTracking()
+            .Where(c => c.ItemId == itemId && c.ToUomId == item.BaseUomId && c.FromUomId != item.BaseUomId)
+            .Select(c => new AllowedUomDto(c.FromUomId, c.FromUom!.Name, c.Factor, false))
+            .ToListAsync(ct);
+
+        // Base first — it is what every form defaults to — then the alternates by name.
+        return converted
+            .OrderBy(u => u.Name)
+            .Prepend(new AllowedUomDto(item.BaseUomId, item.BaseUomName, 1m, true))
+            .ToList();
+    }
+
+    public async Task ValidateUomAsync(Item item, int uomId, CancellationToken ct = default)
+    {
+        if (uomId == item.BaseUomId)
+            return;
+
+        var exists = await _conversions.Query().AsNoTracking()
+            .AnyAsync(c => c.ItemId == item.ItemId && c.FromUomId == uomId && c.ToUomId == item.BaseUomId, ct);
+
+        if (!exists)
+            throw await NoConversionAsync(item, uomId, ct);
+    }
+
+    /// <summary>
+    /// Applies an already-resolved factor to a quantity, rounded to the quantity scale.
+    /// Callers that snapshot a line use this so every base quantity in the system is
+    /// derived and rounded identically.
+    /// </summary>
+    public static decimal ApplyFactor(decimal qty, decimal factor)
+        => Math.Round(qty * factor, QtyScale, MidpointRounding.AwayFromZero);
+
+    /// <summary>
+    /// Restates a base quantity in <paramref name="factor"/>'s UOM. <b>Display only</b> —
+    /// this divides, so it can drift, and its result must never reach a stock or ledger
+    /// write. Conversion toward base is the authoritative direction.
+    /// </summary>
+    public static decimal FromBase(decimal qtyBase, decimal factor)
+        => factor == 0m ? 0m : Math.Round(qtyBase / factor, QtyScale, MidpointRounding.AwayFromZero);
+
+    /// <summary>
+    /// Builds the 422 for an unconvertible UOM. Names the units the item actually accepts,
+    /// because the bare id in the old message told the user nothing about how to recover.
+    /// </summary>
+    private async Task<DomainException> NoConversionAsync(Item item, int uomId, CancellationToken ct)
+    {
+        var attempted = await _uoms.Query().AsNoTracking()
+            .Where(u => u.UomId == uomId)
+            .Select(u => u.Name)
+            .FirstOrDefaultAsync(ct) ?? $"#{uomId}";
+
+        var allowed = await GetAllowedUomsAsync(item.ItemId, ct);
+
+        return new DomainException(ErrorCodes.Validation,
+            $"Item {item.Sku} cannot be transacted in {attempted}. Allowed units: " +
+            $"{string.Join(", ", allowed.Select(u => u.Name))}. " +
+            "Add a UOM conversion on the item to use another unit.", 422);
+    }
 }

Backend/smoke/run_all.py

@@ -25,6 +25,11 @@ SCRIPTS = [
     ("M4b UOM conversion on stock inputs", "m4b_uom_conversion.py"),
     ("M5  terminal receipt + cost pool", "m5_receipt.py"),
     ("M6+M7 leftover / rework / cancel", "m6_m7_leftover_rework_cancel.py"),
+    # UOM engine. These run after the manufacturing scripts because they reuse the
+    # SMOKE-PRD warehouse m4 creates, and they drain it before seeding their own stock.
+    ("UOM conversion direction is enforced", "uom_direction.py"),
+    ("UOM non-base sales consume converted qty", "uom_sales_nonbase.py"),
+    ("UOM cross-unit GRN against a PO", "uom_grn_po_cross.py"),
 ]
 
 SUMMARY = re.compile(r"^(\S+): (\d+)/(\d+) assertions passed")

Backend/smoke/uom_direction.py

@@ -0,0 +1,81 @@
+"""Smoke test — UOM conversions are one-directional, and the API says so.
+
+`UomConverter` looks up exactly one shape, `FromUom -> ToUom = item.BaseUomId`, and never
+inverts a factor. `UpdateUomConversionsAsync` used to accept *any* pair, so saving the more
+natural-reading `base -> BOX` produced a row that returned 200, appeared in the item detail
+response, and was then silently invisible to every consumer — surfacing much later as
+"no UOM conversion" 422 at GRN confirm or stage start, on an item that visibly had one.
+
+  * base -> other is rejected with 422 (the direction that used to save and then not work)
+  * other -> base is accepted
+  * a self-conversion and a base-as-source row are rejected
+  * a zero/negative factor is rejected (UomConverter divides unit cost by it)
+  * changing an item's base UOM while conversions exist is refused rather than orphaning them
+
+    python Backend/smoke/uom_direction.py
+"""
+
+from __future__ import annotations
+
+import sys
+
+from smoke_common import bootstrap
+
+
+def main():
+    c, chk, args = bootstrap(__doc__)
+    print(f"API {args.api}")
+
+    item = c.get("/items?pageSize=1&status=Active").body["items"]
+    if not item:
+        sys.exit("FATAL: no active items.")
+    item = item[0]
+    item_id, base_uom = item["itemId"], item["baseUomId"]
+
+    uoms = c.get("/uoms?pageSize=50").body["items"]
+    other = next((u["uomId"] for u in uoms if u["uomId"] != base_uom), None)
+    if other is None:
+        sys.exit("FATAL: need at least 2 UOMs.")
+    print(f"item={item_id} baseUom={base_uom} otherUom={other}")
+
+    def put(conversions):
+        return c.put(f"/items/{item_id}/uom-conversions", {"conversions": conversions})
+
+    chk.section("1. The correct direction is accepted")
+    ok = put([{"fromUom": other, "toUom": base_uom, "factor": 12}])
+    chk.status("other -> base", ok, 200)
+    if ok.status == 200:
+        chk.check("stored with the base UOM as target", ok.body["conversions"][0]["toUom"], base_uom)
+
+    chk.section("2. The reverse direction is rejected, not silently stored")
+    chk.status("base -> other", put([{"fromUom": base_uom, "toUom": other, "factor": 12}]), 422)
+
+    chk.section("3. Degenerate rows are rejected")
+    chk.status("self-conversion (other -> other)", put([{"fromUom": other, "toUom": other, "factor": 2}]), 422)
+    chk.status("zero factor", put([{"fromUom": other, "toUom": base_uom, "factor": 0}]), 422)
+    chk.status("negative factor", put([{"fromUom": other, "toUom": base_uom, "factor": -3}]), 422)
+
+    chk.section("4. Base UOM cannot be repointed while conversions exist")
+    # Restore a valid conversion first, so the guard has something to protect.
+    put([{"fromUom": other, "toUom": base_uom, "factor": 12}])
+    head = c.get(f"/items/{item_id}")
+    if head.status == 200:
+        body = head.body
+        moved = c.put(f"/items/{item_id}", {
+            "sku": body["sku"], "name": body["name"], "description": body.get("description"),
+            "categoryId": body["categoryId"], "subCategoryId": body.get("subCategoryId"),
+            "brandId": body.get("brandId"),
+            "baseUomId": other,                      # <- the repoint being guarded
+            "defaultVendorId": body.get("defaultVendorId"),
+            "stockNature": body["stockNature"], "trackingMode": body["trackingMode"],
+            "taxClass": body.get("taxClass"), "salePrice": body.get("salePrice"),
+        }, if_match=head.etag)
+        chk.status("change base UOM with conversions defined", moved, 422)
+    else:
+        chk.check("could read the item for the repoint test", head.status, 200)
+
+    return chk.finish("UOM-DIRECTION")
+
+
+if __name__ == "__main__":
+    sys.exit(main())

Backend/smoke/uom_grn_po_cross.py

@@ -0,0 +1,127 @@
+"""Smoke test — receiving in a different UOM from the one ordered.
+
+`GrnService` compared the GRN line's entered quantity against `poLine.Qty - poLine.QtyReceived`
+with no conversion, so a PO for 10 BOX receiving a legitimate 120 base units was rejected
+outright with OVER_RECEIPT_TOLERANCE — a user-visible false failure. It then accrued the GRN's
+quantity into `poLine.QtyReceived` (a PO-UOM field), and the close condition consumed that
+mixed-unit value, so a PO could close early or never close.
+
+Both sides now run on the base pair (`QtyBase` / `QtyReceivedBase`), with `QtyReceived` kept
+as a denormalized display figure only.
+
+  * a receipt in base UOM against a PO raised in BOX is ACCEPTED
+  * the FIFO layer and ledger record the base quantity
+  * `qtyReceivedBase` accrues correctly and the PO reaches FullyReceived
+  * over-receipt beyond tolerance is still rejected, now measured in base units
+
+    python Backend/smoke/uom_grn_po_cross.py
+"""
+
+from __future__ import annotations
+
+import sys
+
+from smoke_common import bootstrap, drain_stock, ensure_vendor
+
+WAREHOUSE_CODE = "SMOKE-PRD"
+FACTOR = 12
+ORDER_BOXES = 10          # -> 120 base units
+RECEIVE_BASE = 120        # the whole order, expressed in base units
+
+
+def main():
+    c, chk, args = bootstrap(__doc__)
+    print(f"API {args.api}")
+
+    wh = next((w["warehouseId"] for w in c.get(f"/warehouses?q={WAREHOUSE_CODE}&pageSize=50").body["items"]
+               if w["code"] == WAREHOUSE_CODE), None)
+    if wh is None:
+        sys.exit("FATAL: run m4_stage_actions.py first (it creates the SMOKE-PRD warehouse).")
+
+    item = next((i for i in c.get("/items?pageSize=20&status=Active").body["items"]
+                 if i["stockNature"] == "Stocked" and i["trackingMode"] == "None"), None)
+    if item is None:
+        sys.exit("FATAL: need a Stocked, untracked item.")
+    base_uom = item["baseUomId"]
+
+    uoms = c.get("/uoms?pageSize=50").body["items"]
+    box_uom = next((u["uomId"] for u in uoms if u["uomId"] != base_uom), None)
+    if box_uom is None:
+        sys.exit("FATAL: need at least 2 UOMs.")
+
+    vendor = ensure_vendor(c)
+    print(f"item={item['itemId']} baseUom={base_uom} boxUom={box_uom} factor={FACTOR}")
+
+    chk.section("1. Conversion + a PO raised in BOX")
+    conv = c.put(f"/items/{item['itemId']}/uom-conversions",
+                 {"conversions": [{"fromUom": box_uom, "toUom": base_uom, "factor": FACTOR}]})
+    chk.status("define BOX -> base conversion", conv, 200)
+    if conv.status != 200:
+        return chk.finish("UOM-GRN-PO")
+
+    po = c.post("/purchase-orders", {
+        "vendorId": vendor,
+        "lines": [{"itemId": item["itemId"], "uomId": box_uom, "warehouseId": wh,
+                   "qty": ORDER_BOXES, "unitPrice": 60, "tax": 0}],
+    })
+    chk.status("create the PO in BOX", po, 201)
+    if po.status != 201:
+        return chk.finish("UOM-GRN-PO")
+
+    po_line = po.body["lines"][0]
+    chk.check("PO line keeps the ordered qty in BOX", float(po_line["qty"]), float(ORDER_BOXES))
+    chk.check("PO line snapshots the base quantity", float(po_line["qtyBase"]), float(ORDER_BOXES * FACTOR))
+    chk.check("PO line snapshots the factor", float(po_line["conversionFactor"]), float(FACTOR))
+
+    drain_stock(c, wh)
+    before = float(c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body["onHand"])
+
+    chk.section("2. Receiving the order in BASE units is accepted")
+    grn = c.post("/grns", {
+        "vendorId": vendor, "warehouseId": wh, "poId": po.body["poId"],
+        "lines": [{"poLineId": po_line["poLineId"], "itemId": item["itemId"],
+                   "uomId": base_uom,                       # <- different UOM from the PO
+                   "qty": RECEIVE_BASE, "unitCost": 5, "discountPct": 0, "vatPct": 0}],
+    })
+    # This is the assertion that fails on the old code: it returned 422 OVER_RECEIPT_TOLERANCE.
+    chk.status("GRN in base UOM against a BOX purchase order", grn, 201)
+    if grn.status != 201:
+        return chk.finish("UOM-GRN-PO")
+
+    confirmed = c.post(f"/grns/{grn.body['grnId']}/confirm")
+    chk.status("confirm the GRN", confirmed, 200)
+    if confirmed.status != 200:
+        return chk.finish("UOM-GRN-PO")
+
+    chk.check("on-hand rose by the base quantity",
+              float(c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body["onHand"]),
+              before + RECEIVE_BASE)
+
+    rows = c.get(f"/stock/ledger?sourceDocType=GRN&sourceDocId={grn.body['grnId']}&pageSize=50").body["items"]
+    chk.check("one GRN ledger row", len(rows), 1)
+    if rows:
+        chk.check("ledger qtyBase is the received base quantity", float(rows[0]["qtyBase"]), float(RECEIVE_BASE))
+
+    chk.section("3. The PO closes on the base pair")
+    reread = c.get(f"/purchase-orders/{po.body['poId']}")
+    chk.status("re-read the PO", reread, 200)
+    if reread.status == 200:
+        rl = reread.body["lines"][0]
+        chk.check("qtyReceivedBase accrued in base units", float(rl["qtyReceivedBase"]), float(RECEIVE_BASE))
+        chk.check("qtyReceived shown back in the PO's own UOM", float(rl["qtyReceived"]), float(ORDER_BOXES))
+        chk.check("PO is FullyReceived", reread.body["status"], "FullyReceived")
+
+    chk.section("4. Over-receipt is still rejected, measured in base")
+    over = c.post("/grns", {
+        "vendorId": vendor, "warehouseId": wh, "poId": po.body["poId"],
+        "lines": [{"poLineId": po_line["poLineId"], "itemId": item["itemId"],
+                   "uomId": base_uom, "qty": RECEIVE_BASE, "unitCost": 5,
+                   "discountPct": 0, "vatPct": 0}],
+    })
+    chk.status("receiving the whole order again", over, 422, "OVER_RECEIPT_TOLERANCE")
+
+    return chk.finish("UOM-GRN-PO")
+
+
+if __name__ == "__main__":
+    sys.exit(main())

Backend/smoke/uom_sales_nonbase.py

@@ -0,0 +1,200 @@
+"""Smoke test — selling in a non-base UOM consumes the converted quantity.
+
+This is the regression test for the UOM engine's worst defect. `SalesPostingService`
+injected `IUomConverter` and never called it: `PostAsync` fed the *entered* line quantity
+straight into `IFifoCostingService.ConsumeAsync`, whose contract is base UOM only. Selling
+2 BOX of a 12-per-box item therefore removed 2 base units instead of 24 and wrote
+`StockLedger.QtyBase = 2` into a column defined as base — overstating stock, understating
+COGS, and drifting the ledger's running balance away from the layer sum.
+
+`m4b_uom_conversion.py` covered exactly the same hazard on the *production* path, which is
+why that path was correct and this one was not. This script closes the gap:
+
+  * an invoice line in a non-base UOM consumes qty x factor base units
+  * the ledger records the BASE quantity
+  * the line still reports the ENTERED qty and UOM, so the printed document says "2 BOX"
+  * the pre-post check reports the shortfall in base units (it compared entered vs base
+    on-hand before, and answered "can post" when it could not)
+  * the same holds for a sales slip, which shares PostAsync
+  * a UOM the item has no conversion for is refused at line creation, not at post
+
+    python Backend/smoke/uom_sales_nonbase.py
+"""
+
+from __future__ import annotations
+
+import sys
+
+from smoke_common import bootstrap, drain_stock, seed_costed_stock
+
+WAREHOUSE_CODE = "SMOKE-PRD"
+FACTOR = 12      # 1 BOX = 12 base units
+SELL_BOXES = 2   # -> 24 base units
+SEED_BASE = 100  # base units on hand before selling
+UNIT_COST = 5.0
+
+
+def main():
+    c, chk, args = bootstrap(__doc__)
+    print(f"API {args.api}")
+
+    wh = next((w["warehouseId"] for w in c.get(f"/warehouses?q={WAREHOUSE_CODE}&pageSize=50").body["items"]
+               if w["code"] == WAREHOUSE_CODE), None)
+    if wh is None:
+        sys.exit("FATAL: run m4_stage_actions.py first (it creates the SMOKE-PRD warehouse).")
+
+    item = next((i for i in c.get("/items?pageSize=20&status=Active").body["items"]
+                 if i["stockNature"] == "Stocked"), None)
+    if item is None:
+        sys.exit("FATAL: need a Stocked item.")
+    base_uom = item["baseUomId"]
+
+    uoms = c.get("/uoms?pageSize=50").body["items"]
+    box_uom = next((u["uomId"] for u in uoms if u["uomId"] != base_uom), None)
+    if box_uom is None:
+        sys.exit("FATAL: need at least 2 UOMs to test conversion.")
+
+    customer = c.get("/customers?pageSize=1").body["items"]
+    if not customer:
+        sys.exit("FATAL: no customers seeded.")
+    customer_id = customer[0]["customerId"]
+
+    print(f"item={item['itemId']} baseUom={base_uom} boxUom={box_uom} factor={FACTOR}")
+
+    # --- fixtures ---------------------------------------------------------
+    chk.section("1. Conversion + known on-hand")
+    conv = c.put(f"/items/{item['itemId']}/uom-conversions",
+                 {"conversions": [{"fromUom": box_uom, "toUom": base_uom, "factor": FACTOR}]})
+    chk.status("define BOX -> base conversion", conv, 200)
+    if conv.status != 200:
+        return chk.finish("UOM-SALES")
+
+    allowed = c.get(f"/items/{item['itemId']}/uoms")
+    chk.status("GET /items/{id}/uoms", allowed, 200)
+    if allowed.status == 200:
+        ids = [u["uomId"] for u in allowed.body]
+        chk.check("allowed UOMs are base + the conversion source", sorted(ids), sorted([base_uom, box_uom]))
+        chk.check("base UOM is flagged and listed first", allowed.body[0]["isBase"], True)
+
+    drain_stock(c, wh)
+    seed_costed_stock(c, wh, [(item["itemId"], base_uom, SEED_BASE, UNIT_COST)])
+    before = float(c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body["onHand"])
+    chk.check(f"on-hand seeded to {SEED_BASE} base units", before, float(SEED_BASE))
+
+    on_hand = c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body
+    chk.check("stock read is labelled with the base UOM", on_hand["baseUomId"], base_uom)
+    chk.check("stock read carries the base UOM name", bool(on_hand["baseUomName"]), True)
+
+    # --- invoice in BOX ---------------------------------------------------
+    chk.section("2. An invoice line entered in BOX")
+    expected_base = SELL_BOXES * FACTOR   # 2 x 12 = 24
+
+    inv = c.post("/sales-invoices", {
+        "customerId": customer_id,
+        "warehouseId": wh,
+        "invoiceType": "B2C",
+        "lines": [{
+            "itemId": item["itemId"], "uomId": box_uom, "warehouseId": wh,
+            "qty": SELL_BOXES, "freeQty": 0, "unitPrice": 100, "allowManualPriceOverride": True,
+            "discountMode": "Percentage", "discountPct": 0, "discountAmount": 0,
+            "discountValue": 0, "taxPct": 0, "isFreeIssue": False,
+        }],
+    })
+    chk.status("create the invoice", inv, 201)
+    if inv.status != 201:
+        return chk.finish("UOM-SALES")
+
+    line = inv.body["lines"][0]
+    chk.check("line keeps the ENTERED qty (prints as 2 BOX)", float(line["qty"]), float(SELL_BOXES))
+    chk.check("line keeps the ENTERED uom", line["uomId"], box_uom)
+
+    check = c.get(f"/sales-invoices/{inv.body['salesInvoiceId']}/posting-check")
+    chk.status("posting check", check, 200)
+    if check.status == 200:
+        chk.check("posting check passes with enough stock", check.body["canPost"], True)
+
+    chk.section("3. Posting consumes the CONVERTED quantity")
+    posted = c.post(f"/sales-invoices/{inv.body['salesInvoiceId']}/post")
+    chk.status("post the invoice", posted, 200)
+    if posted.status != 200:
+        return chk.finish("UOM-SALES")
+
+    after = float(c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body["onHand"])
+    chk.check(f"on-hand fell by {expected_base} base units, not {SELL_BOXES}", after, before - expected_base)
+
+    rows = c.get(f"/stock/ledger?sourceDocType=SINV&sourceDocId={inv.body['salesInvoiceId']}&pageSize=50").body["items"]
+    chk.check("one ledger row for the invoice", len(rows), 1)
+    if rows:
+        chk.check("ledger qtyBase is the CONVERTED quantity", float(rows[0]["qtyBase"]), float(expected_base))
+        chk.check("ledger row is labelled with the base UOM", rows[0]["baseUomId"], base_uom)
+
+    # --- the same on a slip ----------------------------------------------
+    chk.section("4. A sales slip behaves identically (shared PostAsync)")
+    before_slip = float(c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body["onHand"])
+    slip = c.post("/sales-slips", {
+        "customerId": customer_id,
+        "warehouseId": wh,
+        "lines": [{
+            "itemId": item["itemId"], "uomId": box_uom, "warehouseId": wh,
+            "qty": SELL_BOXES, "freeQty": 0, "unitPrice": 100, "allowManualPriceOverride": True,
+            "discountMode": "Percentage", "discountPct": 0, "discountAmount": 0,
+            "discountValue": 0, "taxPct": 0, "isFreeIssue": False,
+        }],
+    })
+    chk.status("create the slip", slip, 201)
+    if slip.status == 201:
+        chk.status("post the slip", c.post(f"/sales-slips/{slip.body['salesSlipId']}/post"), 200)
+        chk.check("slip also consumed the converted quantity",
+                  float(c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body["onHand"]),
+                  before_slip - expected_base)
+
+    # --- snapshot immutability -------------------------------------------
+    chk.section("5. A factor edited after save does not change what posts")
+    draft = c.post("/sales-invoices", {
+        "customerId": customer_id, "warehouseId": wh, "invoiceType": "B2C",
+        "lines": [{
+            "itemId": item["itemId"], "uomId": box_uom, "warehouseId": wh,
+            "qty": 1, "freeQty": 0, "unitPrice": 100, "allowManualPriceOverride": True,
+            "discountMode": "Percentage", "discountPct": 0, "discountAmount": 0,
+            "discountValue": 0, "taxPct": 0, "isFreeIssue": False,
+        }],
+    })
+    if draft.status == 201:
+        # Double the factor *after* the draft is saved.
+        c.put(f"/items/{item['itemId']}/uom-conversions",
+              {"conversions": [{"fromUom": box_uom, "toUom": base_uom, "factor": FACTOR * 2}]})
+        before_snap = float(c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body["onHand"])
+        chk.status("post the pre-existing draft", c.post(f"/sales-invoices/{draft.body['salesInvoiceId']}/post"), 200)
+        chk.check(f"posted the snapshotted {FACTOR}, not the edited {FACTOR * 2}",
+                  float(c.get(f"/stock/on-hand?itemId={item['itemId']}&warehouseId={wh}").body["onHand"]),
+                  before_snap - FACTOR)
+        # Restore for re-runnability.
+        c.put(f"/items/{item['itemId']}/uom-conversions",
+              {"conversions": [{"fromUom": box_uom, "toUom": base_uom, "factor": FACTOR}]})
+    else:
+        chk.check("could create the snapshot-test draft", draft.status, 201)
+
+    # --- unusable UOM refused at entry ------------------------------------
+    chk.section("6. A UOM with no conversion is refused at line creation")
+    third = next((u["uomId"] for u in c.get("/uoms?pageSize=50").body["items"]
+                  if u["uomId"] not in (base_uom, box_uom)), None)
+    if third is None:
+        chk.check("skipped: need a third UOM", True, True)
+    else:
+        bad = c.post("/sales-invoices", {
+            "customerId": customer_id, "warehouseId": wh, "invoiceType": "B2C",
+            "lines": [{
+                "itemId": item["itemId"], "uomId": third, "warehouseId": wh,
+                "qty": 1, "freeQty": 0, "unitPrice": 100, "allowManualPriceOverride": True,
+                "discountMode": "Percentage", "discountPct": 0, "discountAmount": 0,
+                "discountValue": 0, "taxPct": 0, "isFreeIssue": False,
+            }],
+        })
+        # The point is that this fails at CREATE (while the user is editing), not at post.
+        chk.status("invoice line in an unconvertible UOM", bad, 422)
+
+    return chk.finish("UOM-SALES")
+
+
+if __name__ == "__main__":
+    sys.exit(main())

Frontend/erp-system/app/dashboard/procurement/purchase-orders/[id]/page.tsx

@@ -7,6 +7,7 @@ import { AlertTriangle, ArrowLeft, Ban, Check, Plus, Save, Trash2 } from "lucide
 import { isPoEditable, purchaseOrdersApi } from "@/lib/api/purchase-orders"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { itemsApi } from "@/lib/api/items"
+import { uomName } from "@/lib/uom"
 import { uomsApi } from "@/lib/api/uoms"
 import { vendorsApi } from "@/lib/api/vendors"
 import { errorMessage } from "@/lib/error-map"
@@ -111,8 +112,8 @@ export default function PurchaseOrderDetailPage() {
   function itemFor(itemId: number | null) {
     return items.find((i) => i.itemId === itemId) ?? null
   }
-  function uomName(uomId: number) {
+  function uomLabel(uomId: number) {
-    return uoms.find((u) => u.uomId === uomId)?.name ?? `#${uomId}`
+    return uomName(uomId, uoms)
   }
   function warehouseCode(warehouseId: number) {
     return warehouses.find((w) => w.warehouseId === warehouseId)?.code ?? `#${warehouseId}`
@@ -376,7 +377,7 @@ export default function PurchaseOrderDetailPage() {
                   return (
                     <TableRow key={line.key}>
                       <TableCell className="px-3 py-3.5">{item ? `${item.sku} — ${item.name}` : `Item #${line.itemId}`}</TableCell>
-                      <TableCell className="px-3 py-3.5">{line.uomId ? uomName(line.uomId) : "—"}</TableCell>
+                      <TableCell className="px-3 py-3.5">{line.uomId ? uomLabel(line.uomId) : "—"}</TableCell>
                       <TableCell className="px-3 py-3.5">{line.warehouseId ? warehouseCode(line.warehouseId) : "—"}</TableCell>
                       <TableCell className="px-3 py-3.5">{line.qty}</TableCell>
                       <TableCell className="px-3 py-3.5">{line.qtyReceived}</TableCell>

Frontend/erp-system/app/dashboard/procurement/purchase-orders/new/page.tsx

@@ -14,6 +14,8 @@ import { itemsApi } from "@/lib/api/items"
 import { uomsApi } from "@/lib/api/uoms"
 import { errorMessage } from "@/lib/error-map"
 import { validatePoLine } from "@/lib/validations/procurement"
+import { pickerOptions } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { cn } from "@/lib/utils"
 import { generateVendorCode } from "@/lib/vendor-code"
 import { CreatePoLineInput } from "@/types/procurement"
@@ -81,6 +83,7 @@ function NewPurchaseOrderContent() {
   const [lineErrors, setLineErrors] = useState<Record<string, Record<string, string>>>({})
   const [submitError, setSubmitError] = useState<string | null>(null)
   const [submitting, setSubmitting] = useState(false)
+  const allowedUoms = useAllowedUoms()
 
   const [vendorDialogOpen, setVendorDialogOpen] = useState(false)
   const [vName, setVName] = useState("")
@@ -102,7 +105,8 @@ function NewPurchaseOrderContent() {
   useEffect(() => {
     Promise.all([
       loadItems(),
-      uomsApi.list().then((uo) => setUoms(uo.items)),
+      // pageSize: the default page would silently truncate the unit list as the master grows.
+      uomsApi.list({ pageSize: 200 }).then((uo) => setUoms(uo.items)),
       warehousesApi.list().then((wh) => setWarehouses(wh.items)),
       loadVendors(),
     ]).catch((err) => setLoadError(errorMessage(err)))
@@ -428,7 +432,17 @@ function NewPurchaseOrderContent() {
                               <div className="flex h-11 items-center truncate text-base" title={item ? `${item.sku} — ${item.name}` : undefined}>{item ? `${item.sku} — ${item.name}` : `Item #${line.itemId}`}</div>
                             ) : (
                               <>
-                                <Select<number | null> value={line.itemId} onValueChange={(v) => updateLine(line.key, { itemId: v })}>
+                                <Select<number | null>
+                                  value={line.itemId}
+                                  onValueChange={(v) => {
+                                    // Reset to the item's base unit and load its allowed units.
+                                    allowedUoms.load(v)
+                                    updateLine(line.key, {
+                                      itemId: v,
+                                      uomId: (items ?? []).find((i) => i.itemId === v)?.baseUomId ?? null,
+                                    })
+                                  }}
+                                >
                                   <SelectTrigger
                                     className="h-11! w-full text-base"
                                     aria-invalid={!!errors.itemId}
@@ -454,7 +468,7 @@ function NewPurchaseOrderContent() {
                                 <SelectValue placeholder="UOM" />
                               </SelectTrigger>
                               <SelectContent>
-                                {(uoms ?? []).map((u) => (
+                                {pickerOptions(allowedUoms.get(line.itemId), uoms ?? []).map((u) => (
                                   <SelectItem key={u.uomId} value={u.uomId} className="text-base">
                                     {u.name}
                                   </SelectItem>

Frontend/erp-system/app/dashboard/production/templates/[id]/StageEditorPanel.tsx

@@ -3,6 +3,8 @@
 import { Plus, Trash2 } from "lucide-react"
 
 import { cn } from "@/lib/utils"
+import { pickerOptions } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { CustomFieldType, StageInputSource } from "@/types/production"
 import { ItemListItem, Uom } from "@/types/master-data"
 import {
@@ -46,7 +48,8 @@ function QtyRow({
 }: {
   qty: number
   uomId: number | null
-  uoms: Uom[]
+  /** Already narrowed to the row's item by the caller — see `pickerOptions`. */
+  uoms: { uomId: number; name: string }[]
   readOnly: boolean
   onQtyChange: (qty: number) => void
   onUomChange: (uomId: number) => void
@@ -125,9 +128,12 @@ export function StageEditorPanel({
     updateInput(localId, source === "Stock" ? { source, fromOutputKey: null } : { source, itemId: null })
   }
 
+  const allowedUoms = useAllowedUoms()
+
   /** Default the UOM to the item's base unit — right most of the time, still overridable. */
   function pickInputItem(input: BuilderInput, itemId: number) {
     const item = items.find((i) => i.itemId === itemId)
+    allowedUoms.load(itemId)
     updateInput(input.localId, { itemId, uomId: input.uomId ?? item?.baseUomId ?? null })
   }
 
@@ -146,6 +152,7 @@ export function StageEditorPanel({
   /** The terminal output's name mirrors the finished item, so the two can't drift apart. */
   function pickOutputItem(output: BuilderOutput, itemId: number) {
     const item = items.find((i) => i.itemId === itemId)
+    allowedUoms.load(itemId)
     updateOutput(output.key, {
       itemId,
       name: item?.name ?? output.name,
@@ -288,7 +295,7 @@ export function StageEditorPanel({
                     <QtyRow
                       qty={input.qtyPerBatch}
                       uomId={input.uomId}
-                      uoms={uoms}
+                      uoms={pickerOptions(allowedUoms.get(input.itemId), uoms)}
                       readOnly={readOnly}
                       onQtyChange={(qtyPerBatch) => updateInput(input.localId, { qtyPerBatch })}
                       onUomChange={(uomId) => updateInput(input.localId, { uomId })}
@@ -348,7 +355,7 @@ export function StageEditorPanel({
                   <QtyRow
                     qty={output.qtyPerBatch}
                     uomId={output.uomId}
-                    uoms={uoms}
+                    uoms={pickerOptions(allowedUoms.get(output.itemId), uoms)}
                     readOnly={readOnly}
                     onQtyChange={(qtyPerBatch) => updateOutput(output.key, { qtyPerBatch })}
                     onUomChange={(uomId) => updateOutput(output.key, { uomId })}

Frontend/erp-system/app/dashboard/products/[id]/UomConversionsPanel.tsx

@@ -0,0 +1,193 @@
+"use client"
+
+// Per-item UOM conversion editor (FR-MD-02).
+//
+// Until this panel existed, `PUT /items/{id}/uom-conversions` had no caller anywhere in the
+// app: conversions were typed, and had an API client method, but no screen could create one.
+// That made every non-base UOM unusable — a user could pick "Box-12" on an invoice line and
+// only discover at post time that the item had no conversion for it.
+//
+// Direction is fixed and not user-editable: a row always converts **into** the item's base
+// UOM. The server enforces that (`toUom` must equal `baseUomId`), because the conversion
+// engine only ever looks up `<other> → base` and never inverts a factor. Rendering `toUom`
+// as fixed text rather than a second picker is what keeps the two in step.
+import { useState } from "react"
+import { Plus, Save, Trash2 } from "lucide-react"
+
+import { itemsApi } from "@/lib/api/items"
+import { errorMessage } from "@/lib/error-map"
+import { validateConversionLine } from "@/lib/validations/master-data"
+import { UomConversion } from "@/types/master-data"
+
+import { Button } from "@/components/ui/button"
+import { Input } from "@/components/ui/input"
+import { Label } from "@/components/ui/label"
+import { FieldError } from "@/components/ui/field"
+import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from "@/components/ui/select"
+import { Table, TableBody, TableCell, TableHead, TableHeader, TableRow } from "@/components/ui/table"
+import { toast } from "@/components/ui/toast"
+
+interface DraftRow {
+  key: string
+  fromUom: number | null
+  factor: string
+}
+
+interface Props {
+  itemId: number
+  baseUomId: number
+  conversions: UomConversion[]
+  uoms: { uomId: number; name: string }[]
+  /** Lets the parent refresh the item so `conversions` stays in sync after a save. */
+  onSaved: () => void
+}
+
+let rowSeq = 0
+const nextKey = () => `conv-${rowSeq++}`
+
+export function UomConversionsPanel({ itemId, baseUomId, conversions, uoms, onSaved }: Props) {
+  const [rows, setRows] = useState<DraftRow[]>(() =>
+    conversions.map((c) => ({ key: nextKey(), fromUom: c.fromUom, factor: String(c.factor) })),
+  )
+  const [errors, setErrors] = useState<Record<string, Record<string, string>>>({})
+  const [saving, setSaving] = useState(false)
+
+  const baseName = uoms.find((u) => u.uomId === baseUomId)?.name ?? "base UOM"
+  // The base UOM converts to itself implicitly; offering it here would only produce a 422.
+  const selectableUoms = uoms.filter((u) => u.uomId !== baseUomId)
+
+  function updateRow(key: string, patch: Partial<DraftRow>) {
+    setRows((prev) => prev.map((r) => (r.key === key ? { ...r, ...patch } : r)))
+  }
+
+  function addRow() {
+    setRows((prev) => [...prev, { key: nextKey(), fromUom: null, factor: "" }])
+  }
+
+  function removeRow(key: string) {
+    setRows((prev) => prev.filter((r) => r.key !== key))
+  }
+
+  async function handleSave() {
+    const nextErrors: Record<string, Record<string, string>> = {}
+    for (const row of rows) {
+      const rowErrors = validateConversionLine({ fromUom: row.fromUom, toUom: baseUomId, factor: row.factor })
+      if (Object.keys(rowErrors).length > 0) nextErrors[row.key] = rowErrors
+    }
+
+    // The unique index is on (item, from, to); catching it here beats a 400 from the server.
+    const chosen = rows.map((r) => r.fromUom).filter((u): u is number => u !== null)
+    const duplicates = chosen.filter((u, i) => chosen.indexOf(u) !== i)
+    for (const row of rows) {
+      if (row.fromUom !== null && duplicates.includes(row.fromUom)) {
+        nextErrors[row.key] = { ...nextErrors[row.key], fromUom: "One conversion per unit" }
+      }
+    }
+
+    setErrors(nextErrors)
+    if (Object.keys(nextErrors).length > 0) return
+
+    setSaving(true)
+    try {
+      // A full replace, matching the server's upsert semantics: an empty list clears them all,
+      // which is also how a user detaches conversions before changing the base UOM.
+      await itemsApi.updateUomConversions(itemId, {
+        conversions: rows.map((r) => ({ fromUom: r.fromUom as number, toUom: baseUomId, factor: Number(r.factor) })),
+      })
+      toast.success("Conversions saved", `${rows.length} conversion(s) against ${baseName}`)
+      onSaved()
+    } catch (err) {
+      toast.error("Could not save conversions", errorMessage(err))
+    } finally {
+      setSaving(false)
+    }
+  }
+
+  return (
+    <div className="flex flex-col gap-4 rounded-lg border border-border bg-card p-5">
+      <div className="flex flex-col gap-1">
+        <Label className="text-base font-semibold">UOM conversions</Label>
+        <p className="text-sm text-muted-foreground">
+          Units this item can be bought, sold, or produced in besides {baseName}. Each row says how many{" "}
+          {baseName} one of that unit is worth — a Box of 12 pieces is a factor of 12. Stock is always stored in{" "}
+          {baseName}.
+        </p>
+      </div>
+
+      {rows.length === 0 ? (
+        <p className="text-sm text-muted-foreground">
+          No conversions — this item can only be transacted in {baseName}.
+        </p>
+      ) : (
+        <Table>
+          <TableHeader>
+            <TableRow>
+              <TableHead className="w-64">Unit</TableHead>
+              <TableHead className="w-40 text-right">Factor</TableHead>
+              <TableHead>Converts to</TableHead>
+              <TableHead className="w-16" />
+            </TableRow>
+          </TableHeader>
+          <TableBody>
+            {rows.map((row) => (
+              <TableRow key={row.key}>
+                <TableCell>
+                  <Select<number | null>
+                    value={row.fromUom}
+                    onValueChange={(v) => updateRow(row.key, { fromUom: v })}
+                    items={selectableUoms.map((u) => ({ label: u.name, value: u.uomId }))}
+                  >
+                    <SelectTrigger className="h-11! w-full text-base" aria-invalid={!!errors[row.key]?.fromUom}>
+                      <SelectValue placeholder="Select unit" />
+                    </SelectTrigger>
+                    <SelectContent>
+                      {selectableUoms.map((u) => (
+                        <SelectItem key={u.uomId} value={u.uomId} className="text-base">
+                          {u.name}
+                        </SelectItem>
+                      ))}
+                    </SelectContent>
+                  </Select>
+                  <FieldError errors={[errors[row.key]?.fromUom ? { message: errors[row.key].fromUom } : undefined]} />
+                </TableCell>
+                <TableCell className="text-right">
+                  <Input
+                    type="number"
+                    min="0"
+                    step="0.000001"
+                    value={row.factor}
+                    onChange={(e) => updateRow(row.key, { factor: e.target.value })}
+                    aria-invalid={!!errors[row.key]?.factor}
+                    className="text-right"
+                  />
+                  <FieldError errors={[errors[row.key]?.factor ? { message: errors[row.key].factor } : undefined]} />
+                </TableCell>
+                <TableCell className="text-base text-muted-foreground">
+                  {row.factor && Number(row.factor) > 0
+                    ? `1 ${uoms.find((u) => u.uomId === row.fromUom)?.name ?? "unit"} = ${Number(row.factor)} ${baseName}`
+                    : baseName}
+                </TableCell>
+                <TableCell>
+                  <Button variant="ghost" size="icon" onClick={() => removeRow(row.key)} aria-label="Remove conversion">
+                    <Trash2 className="size-5 text-destructive" />
+                  </Button>
+                </TableCell>
+              </TableRow>
+            ))}
+          </TableBody>
+        </Table>
+      )}
+
+      <div className="flex justify-between gap-3">
+        <Button variant="outline" onClick={addRow}>
+          <Plus className="size-5" />
+          Add conversion
+        </Button>
+        <Button onClick={handleSave} disabled={saving}>
+          <Save className="size-5" />
+          {saving ? "Saving…" : "Save conversions"}
+        </Button>
+      </div>
+    </div>
+  )
+}

Frontend/erp-system/app/dashboard/products/[id]/page.tsx

@@ -11,9 +11,12 @@ import { uomsApi } from "@/lib/api/uoms"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { errorMessage, fieldErrors } from "@/lib/error-map"
 import { validateItemForm } from "@/lib/validations/master-data"
+import { uomName } from "@/lib/uom"
 import { cn } from "@/lib/utils"
 import { Item, StockNature, TrackingMode } from "@/types/master-data"
 
+import { UomConversionsPanel } from "./UomConversionsPanel"
+
 import { Badge } from "@/components/ui/badge"
 import { Button, buttonVariants } from "@/components/ui/button"
 import { Input } from "@/components/ui/input"
@@ -92,7 +95,9 @@ export default function ItemDetailPage() {
   useEffect(() => {
     if (!Number.isFinite(itemId)) return
     load()
-    Promise.all([categoriesApi.list(), uomsApi.list(), warehousesApi.list({ pageSize: 200 })])
+    // pageSize matters here: the default page would silently truncate the UOM list as the
+    // master grows, hiding units from the base-UOM picker and the conversions editor.
+    Promise.all([categoriesApi.list(), uomsApi.list({ pageSize: 200 }), warehousesApi.list({ pageSize: 200 })])
       .then(([cat, uo, wh]) => {
         setCategories(cat.items)
         setUoms(uo.items)
@@ -155,9 +160,6 @@ export default function ItemDetailPage() {
     }
   }
 
-  function uomName(uomId: number) {
-    return uoms.find((u) => u.uomId === uomId)?.name ?? `#${uomId}`
-  }
 
   if (loadError && !item) {
     return (
@@ -321,8 +323,19 @@ export default function ItemDetailPage() {
       </div>
 
       <p className="text-sm text-muted-foreground">
-        {uomName(item.baseUomId)} is the base UOM — every transaction converts to and stores quantities in base UOM (FR-MD-03).
+        {uomName(item.baseUomId, uoms)} is the base UOM — every transaction converts to and stores quantities in base UOM (FR-MD-03).
       </p>
+
+      <UomConversionsPanel
+        // Remount when the saved set changes so the editor's draft rows re-seed from the
+        // server's response rather than keeping stale local state after a save or reload.
+        key={item.conversions.map((c) => `${c.conversionId}:${c.factor}`).join("|")}
+        itemId={item.itemId}
+        baseUomId={item.baseUomId}
+        conversions={item.conversions}
+        uoms={uoms}
+        onSaved={load}
+      />
     </div>
   )
 }

Frontend/erp-system/app/dashboard/receiving/grn/[id]/page.tsx

@@ -8,6 +8,7 @@ import { CheckCircle2, PackageCheck, PackageX, ShieldAlert, XCircle } from "luci
 import { grnsApi } from "@/lib/api/grns"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { itemsApi } from "@/lib/api/items"
+import { uomName } from "@/lib/uom"
 import { uomsApi } from "@/lib/api/uoms"
 import { errorMessage } from "@/lib/error-map"
 import { cn } from "@/lib/utils"
@@ -57,7 +58,7 @@ export default function GrnDetailPage() {
     return items.find((i) => i.itemId === itemId)
   }
   function uomFor(uomId: number) {
-    return uoms.find((u) => u.uomId === uomId)?.name ?? `#${uomId}`
+    return uomName(uomId, uoms)
   }
   function binFor(binId: number | null) {
     if (!binId) return "—"

Frontend/erp-system/app/dashboard/receiving/grn/new/page.tsx

@@ -13,6 +13,8 @@ import { itemsApi } from "@/lib/api/items"
 import { uomsApi } from "@/lib/api/uoms"
 import { errorMessage } from "@/lib/error-map"
 import { validateLine, splitSerials, grnHeaderSchema } from "@/lib/validations/grn"
+import { basePreview, pickerOptions, uomName } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { cn } from "@/lib/utils"
 import { CreateGrnLineInput, HoldStatus } from "@/types/grn"
 import { PurchaseOrder, PurchaseOrderSummary } from "@/types/procurement"
@@ -108,12 +110,14 @@ export default function NewGrnPage() {
   const [submitError, setSubmitError] = useState<string | null>(null)
   const [submitting, setSubmitting] = useState(false)
   const [refreshingItems, setRefreshingItems] = useState(false)
+  const allowedUoms = useAllowedUoms()
 
   useEffect(() => {
     Promise.all([
       warehousesApi.list(),
       itemsApi.list({ pageSize: 200, status: "Active" }),
-      uomsApi.list(),
+      // pageSize: without it the default page silently truncates the unit list.
+      uomsApi.list({ pageSize: 200 }),
       vendorsApi.list({ pageSize: 200, status: "Active" }),
       purchaseOrdersApi.list({ pageSize: 200 }),
     ])
@@ -155,13 +159,18 @@ export default function NewGrnPage() {
     setHeaderError(null)
     try {
       const po: PurchaseOrder = await purchaseOrdersApi.get(nextPoId)
-      const openLines = po.lines.filter((l) => l.qtyReceived < l.qty)
+      // Open-ness is decided on the base pair, matching the server's close condition —
+      // qty/qtyReceived are PO-UOM display figures derived by division and can drift.
+      const openLines = po.lines.filter((l) => l.qtyReceivedBase < l.qtyBase)
       if (openLines.length === 0) {
         setHeaderError("This purchase order has no open (unreceived) lines.")
         setLines([])
         return
       }
       setWarehouseId((prev) => prev ?? openLines[0].warehouseId)
+      // The receiving lines keep the PO's UOM, so preload each item's allowed units for the
+      // few lines a user may switch to a different pack size.
+      openLines.forEach((l) => allowedUoms.load(l.itemId))
       setLines(
         openLines.map(
           (l): DraftLine => ({
@@ -469,7 +478,16 @@ export default function NewGrnPage() {
                               </div>
                             ) : (
                               <>
-                                <Select<number | null> value={line.itemId} onValueChange={(v) => updateLine(line.key, { itemId: v })}>
+                                <Select<number | null>
+                                  value={line.itemId}
+                                  onValueChange={(v) => {
+                                    // Reset the unit to the item's base and load its allowed
+                                    // units — a UOM carried over from the previous item would
+                                    // usually have no conversion for the new one.
+                                    allowedUoms.load(v)
+                                    updateLine(line.key, { itemId: v, uomId: itemFor(v)?.baseUomId ?? null })
+                                  }}
+                                >
                                   <SelectTrigger className="h-11! w-full text-base" aria-invalid={!!errors.itemId}>
                                     <SelectValue placeholder="Select item" />
                                   </SelectTrigger>
@@ -488,7 +506,7 @@ export default function NewGrnPage() {
                           <TableCell className="px-3 py-3 align-top">
                             {line.poLineId ? (
                               <div className="flex h-11 items-center text-base">
-                                {uoms?.find((u) => u.uomId === line.uomId)?.name ?? `#${line.uomId}`}
+                                {uomName(line.uomId, uoms ?? [])}
                               </div>
                             ) : (
                               <>
@@ -497,7 +515,7 @@ export default function NewGrnPage() {
                                     <SelectValue placeholder="UOM" />
                                   </SelectTrigger>
                                   <SelectContent>
-                                    {(uoms ?? []).map((u) => (
+                                    {pickerOptions(allowedUoms.get(line.itemId), uoms ?? []).map((u) => (
                                       <SelectItem key={u.uomId} value={u.uomId} className="text-base">
                                         {u.name}
                                       </SelectItem>
@@ -532,6 +550,12 @@ export default function NewGrnPage() {
                               onChange={(e) => updateLine(line.key, { qty: e.target.value })}
                               className="h-11 text-base"
                             />
+                            {/* Shows what will actually hit stock when receiving in a pack unit. */}
+                            {basePreview(Number(line.qty), line.uomId, allowedUoms.get(line.itemId) ?? []) && (
+                              <p className="mt-1 text-xs text-muted-foreground">
+                                {basePreview(Number(line.qty), line.uomId, allowedUoms.get(line.itemId) ?? [])}
+                              </p>
+                            )}
                             <FieldError errors={[errors.qty ? { message: errors.qty } : undefined]} />
                           </TableCell>
                           <TableCell className="px-3 py-3 align-top">

Frontend/erp-system/app/dashboard/sales/bundles/[id]/page.tsx

@@ -9,6 +9,8 @@ import { bundleApi } from "@/lib/api/bundles"
 import { itemsApi } from "@/lib/api/items"
 import { uomsApi } from "@/lib/api/uoms"
 import { errorMessage } from "@/lib/error-map"
+import { basePreview, pickerOptions } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { Button, buttonVariants } from "@/components/ui/button"
 import { Badge } from "@/components/ui/badge"
 import { Input } from "@/components/ui/input"
@@ -55,6 +57,7 @@ export default function BundleSaleDetailPage() {
   const bundleSaleId = Number(params.id)
   const [bundle, setBundle] = useState<BundleSale | null>(null)
   const [editing, setEditing] = useState(false)
+  const allowedUoms = useAllowedUoms()
   const [templates, setTemplates] = useState<BundleSaleTemplateSummary[]>([])
   const [template, setTemplate] = useState<BundleSaleTemplate | null>(null)
   const [customers, setCustomers] = useState<Customer[]>([])
@@ -102,12 +105,16 @@ export default function BundleSaleDetailPage() {
         setTemplateId(data.bundleSaleTemplateId)
         setBundleName(data.bundleName)
         setBundlePrice(data.bundlePrice)
+        data.lines.forEach((line) => allowedUoms.load(line.itemId))
         setLines(
           data.lines.map((line) => ({
             key: `${line.bundleSaleLineId}`,
             bundleSaleTemplateLineId: line.bundleSaleLineId,
             itemId: line.itemId,
-            uomId: items.find((candidate) => candidate.itemId === line.itemId)?.baseUomId ?? line.uomId,
+            // Keep the UOM the line was saved with. This used to be forced back to the
+            // item's base because the server rewrote it that way; it now preserves what
+            // was entered, so overriding here would discard the user's choice.
+            uomId: line.uomId,
             warehouseId: line.warehouseId,
             qty: line.qty,
             unitPrice: line.unitPrice,
@@ -377,6 +384,7 @@ export default function BundleSaleDetailPage() {
                       <Select value={line.itemId ? String(line.itemId) : "all"} onValueChange={(v) => {
                         const itemId = Number(v)
                         const item = items.find((candidate) => candidate.itemId === itemId)
+                        allowedUoms.load(itemId)
                         updateLine(line.key, { itemId, uomId: item?.baseUomId ?? line.uomId, unitPrice: item?.salePrice ?? line.unitPrice })
                       }} disabled={!editing || !isDraft}>
                         <SelectTrigger className="h-9 text-sm">
@@ -392,12 +400,14 @@ export default function BundleSaleDetailPage() {
                       </Select>
                     </TableCell>
                     <TableCell className="min-w-40">
-                      <Select value={line.uomId ? String(line.uomId) : "all"} onValueChange={(v) => updateLine(line.key, { uomId: v === "all" ? 0 : Number(v) })} disabled>
+                      {/* Enabled now that bundle lines keep the entered UOM (the server
+                          snapshots QtyBase beside it instead of overwriting Qty). */}
+                      <Select value={line.uomId ? String(line.uomId) : "all"} onValueChange={(v) => updateLine(line.key, { uomId: v === "all" ? 0 : Number(v) })} disabled={!editing || !isDraft || !line.itemId}>
                         <SelectTrigger className="h-9 text-sm">
                           <SelectValue placeholder="Select UOM" />
                         </SelectTrigger>
                         <SelectContent>
-                          {uoms.map((uom) => (
+                          {pickerOptions(allowedUoms.get(line.itemId), uoms).map((uom) => (
                             <SelectItem key={uom.uomId} value={String(uom.uomId)}>
                               {uom.name}
                             </SelectItem>
@@ -405,7 +415,12 @@ export default function BundleSaleDetailPage() {
                         </SelectContent>
                       </Select>
                     </TableCell>
-                    <TableCell className="text-right w-28"><Input type="number" min="0" step="0.01" value={line.qty} onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) })} disabled={!editing || !isDraft} className="text-right" /></TableCell>
+                    <TableCell className="text-right w-28">
+                      <Input type="number" min="0" step="0.01" value={line.qty} onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) })} disabled={!editing || !isDraft} className="text-right" />
+                      {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? []) && (
+                        <p className="mt-1 text-xs text-muted-foreground">{basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? [])}</p>
+                      )}
+                    </TableCell>
                     <TableCell className="text-right w-32"><Input type="number" min="0" step="0.01" value={line.unitPrice} onChange={(e) => updateLine(line.key, { unitPrice: Number(e.target.value) })} disabled={!editing || !isDraft} className="text-right" /></TableCell>
                     <TableCell className="text-right">
                       {editing && isDraft ? (

Frontend/erp-system/app/dashboard/sales/bundles/new/page.tsx

@@ -12,6 +12,8 @@ import { uomsApi } from "@/lib/api/uoms"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { usersApi } from "@/lib/api/users"
 import { errorMessage } from "@/lib/error-map"
+import { basePreview, pickerOptions } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { Button, buttonVariants } from "@/components/ui/button"
 import { Input } from "@/components/ui/input"
 import { Label } from "@/components/ui/label"
@@ -61,6 +63,7 @@ function NewBundleSaleContent() {
   const [loading, setLoading] = useState(true)
   const [saving, setSaving] = useState(false)
   const [submitError, setSubmitError] = useState<string | null>(null)
+  const allowedUoms = useAllowedUoms()
 
   useEffect(() => {
     Promise.all([
@@ -94,11 +97,12 @@ function NewBundleSaleContent() {
       setLines(
         res.lines.length > 0
           ? res.lines.map((line) => {
-              const item = items.find((candidate) => candidate.itemId === line.itemId)
+              // Load each component's allowed units so the picker narrows for template-seeded rows too.
-              return createBlankLine({
+              allowedUoms.load(line.itemId)
-                ...line,
+              // Keep the template's own UOM. Forcing it to the item's base while keeping the
-                uomId: item?.baseUomId ?? line.uomId,
+              // template's unitPrice would leave qty and price in different units, and
-              })
+              // componentSubtotal below multiplies the two.
+              return createBlankLine({ ...line, uomId: line.uomId })
             })
           : [createBlankLine()]
       )
@@ -259,6 +263,7 @@ function NewBundleSaleContent() {
                     <Select value={line.itemId ? String(line.itemId) : "all"} onValueChange={(v) => {
                         const itemId = Number(v)
                         const item = items.find((candidate) => candidate.itemId === itemId)
+                        allowedUoms.load(itemId)
                         updateLine(line.key, {
                           itemId,
                           uomId: item?.baseUomId ?? line.uomId,
@@ -278,12 +283,14 @@ function NewBundleSaleContent() {
                     </Select>
                   </TableCell>
                   <TableCell className="min-w-40">
-                      <Select value={line.uomId ? String(line.uomId) : "all"} onValueChange={(v) => updateLine(line.key, { uomId: v === "all" ? 0 : Number(v) })} disabled>
+                      {/* Enabled now that bundle lines keep the entered UOM: the server
+                          snapshots the base quantity beside it instead of overwriting qty. */}
+                      <Select value={line.uomId ? String(line.uomId) : "all"} onValueChange={(v) => updateLine(line.key, { uomId: v === "all" ? 0 : Number(v) })} disabled={!line.itemId}>
                       <SelectTrigger className="h-9 text-sm">
                         <SelectValue placeholder="Select UOM" />
                       </SelectTrigger>
                       <SelectContent>
-                        {uoms.map((uom) => (
+                        {pickerOptions(allowedUoms.get(line.itemId), uoms).map((uom) => (
                           <SelectItem key={uom.uomId} value={String(uom.uomId)}>
                             {uom.name}
                           </SelectItem>
@@ -291,7 +298,14 @@ function NewBundleSaleContent() {
                       </SelectContent>
                     </Select>
                   </TableCell>
-                  <TableCell className="text-right w-28"><Input type="number" min="0" step="0.01" value={line.qty} onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) })} className="text-right" /></TableCell>
+                  <TableCell className="text-right w-28">
+                    <Input type="number" min="0" step="0.01" value={line.qty} onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) })} className="text-right" />
+                    {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? []) && (
+                      <p className="mt-1 text-xs text-muted-foreground">
+                        {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? [])}
+                      </p>
+                    )}
+                  </TableCell>
                   <TableCell className="text-right w-32"><Input type="number" min="0" step="0.01" value={line.unitPrice} onChange={(e) => updateLine(line.key, { unitPrice: Number(e.target.value) })} className="text-right" /></TableCell>
                   <TableCell className="text-right">
                     <button

Frontend/erp-system/app/dashboard/sales/free-issues/new/page.tsx

@@ -10,6 +10,8 @@ import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from "@
 import { Table, TableBody, TableCell, TableHead, TableHeader, TableRow } from "@/components/ui/table"
 import { cn } from "@/lib/utils"
 import { errorMessage } from "@/lib/error-map"
+import { pickerOptions, uomName as resolveUomName } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { salesApi } from "@/lib/api/sales"
 import { customersApi } from "@/lib/api/customers"
 import { itemsApi } from "@/lib/api/items"
@@ -70,6 +72,7 @@ export default function NewFreeIssuePage() {
   const [loading, setLoading] = useState(true)
   const [saving, setSaving] = useState(false)
   const [error, setError] = useState<string | null>(null)
+  const allowedUoms = useAllowedUoms()
 
   async function refreshRows() {
     const list = await salesApi.listFreeIssues({ pageSize: 50 })
@@ -88,7 +91,7 @@ export default function NewFreeIssuePage() {
           warehouseName: warehouse?.name ?? `Warehouse ${detail.data.warehouseId}`,
           itemName: item?.name ?? firstLine?.description ?? "—",
           itemSku: item?.sku ?? `SKU-${firstLine?.itemId ?? 0}`,
-          uomName: uom?.name ?? `UOM ${firstLine?.uomId ?? 0}`,
+          uomName: uom?.name ?? resolveUomName(firstLine?.uomId, uoms),
           qty: firstLine?.qty ?? 0,
           freeQty: firstLine?.freeQty ?? 0,
         } satisfies FreeIssueRow
@@ -146,11 +149,13 @@ export default function NewFreeIssuePage() {
 
   function selectItem(key: string, itemId: number) {
     const item = items.find((candidate) => candidate.itemId === itemId)
+    allowedUoms.load(itemId)
     updateLine(key, { itemId, uomId: item?.baseUomId ?? 0 })
   }
 
   function selectEditingItem(key: string, itemId: number) {
     const item = items.find((candidate) => candidate.itemId === itemId)
+    allowedUoms.load(itemId)
     updateEditingLine(key, { itemId, uomId: item?.baseUomId ?? 0 })
   }
 
@@ -325,7 +330,7 @@ export default function NewFreeIssuePage() {
                           <SelectValue placeholder="UOM" />
                         </SelectTrigger>
                         <SelectContent>
-                          {uoms.map((u) => (
+                          {pickerOptions(allowedUoms.get(line.itemId), uoms).map((u) => (
                             <SelectItem key={u.uomId} value={String(u.uomId)}>
                               {u.name}
                             </SelectItem>
@@ -390,7 +395,7 @@ export default function NewFreeIssuePage() {
                           <SelectValue placeholder="UOM" />
                         </SelectTrigger>
                         <SelectContent>
-                          {uoms.map((u) => (
+                          {pickerOptions(allowedUoms.get(line.itemId), uoms).map((u) => (
                             <SelectItem key={u.uomId} value={String(u.uomId)}>
                               {u.name}
                             </SelectItem>

Frontend/erp-system/app/dashboard/sales/invoices/[id]/page.tsx

@@ -11,6 +11,8 @@ import { itemsApi } from "@/lib/api/items"
 import { uomsApi } from "@/lib/api/uoms"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { errorMessage } from "@/lib/error-map"
+import { basePreview, pickerOptions, uomName } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { cn } from "@/lib/utils"
 import { getSuggestedUnitPrice } from "@/lib/sales-line-utils"
 import { Customer } from "@/types/customers"
@@ -75,6 +77,7 @@ export default function SalesInvoiceDetailPage({ params }: { params: Promise<{ i
   const [error, setError] = useState<string | null>(null)
   const [saving, setSaving] = useState(false)
   const [busy, setBusy] = useState<"post" | "cancel" | null>(null)
+  const allowedUoms = useAllowedUoms()
 
   useEffect(() => {
     if (!Number.isFinite(invoiceId)) {
@@ -99,6 +102,9 @@ export default function SalesInvoiceDetailPage({ params }: { params: Promise<{ i
         setCustomerId(doc.data.customerId)
         setWarehouseId(doc.data.warehouseId)
         setInvoiceType(doc.data.invoiceType)
+        // Preload allowed units for the items already on the document, so editing an
+        // existing line offers the narrowed list immediately.
+        doc.data.lines.forEach((line) => allowedUoms.load(line.itemId))
         setLines(
           doc.data.lines.map((line) => ({
             key: String(line.salesInvoiceLineId),
@@ -150,6 +156,7 @@ export default function SalesInvoiceDetailPage({ params }: { params: Promise<{ i
 
   function selectItem(key: string, itemId: number) {
     const item = items.find((candidate) => candidate.itemId === itemId)
+    allowedUoms.load(itemId)
     updateLine(key, {
       itemId,
       uomId: item?.baseUomId ?? 0,
@@ -353,7 +360,7 @@ export default function SalesInvoiceDetailPage({ params }: { params: Promise<{ i
                     <div className="font-medium text-foreground">{line.description}</div>
                     <div className="text-xs text-muted-foreground">SKU {items.find((i) => i.itemId === line.itemId)?.sku ?? `#${line.itemId}`}</div>
                   </td>
-                  <td className="px-4 py-3">{uoms.find((u) => u.uomId === line.uomId)?.name ?? `#${line.uomId}`}</td>
+                  <td className="px-4 py-3">{uomName(line.uomId, uoms)}</td>
                   <td className="px-4 py-3 text-right">{line.qty.toFixed(0)}</td>
                   <td className="px-4 py-3 text-right">{line.freeQty > 0 ? line.freeQty.toFixed(0) : "—"}</td>
                   <td className="px-4 py-3 text-right">{money.format(line.unitPrice)}</td>
@@ -503,7 +510,7 @@ export default function SalesInvoiceDetailPage({ params }: { params: Promise<{ i
                             className="h-10 w-full rounded-lg border border-input bg-background px-3 text-sm"
                           >
                             <option value="">UOM</option>
-                            {uoms.map((u) => (
+                            {pickerOptions(allowedUoms.get(line.itemId), uoms).map((u) => (
                               <option key={u.uomId} value={String(u.uomId)}>
                                 {u.name}
                               </option>
@@ -519,6 +526,11 @@ export default function SalesInvoiceDetailPage({ params }: { params: Promise<{ i
                             onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) || 0 })}
                             className="h-10 w-full rounded-lg border border-input bg-background px-3 text-right font-mono tabular-nums text-sm"
                           />
+                          {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? []) && (
+                            <p className="mt-1 text-xs text-muted-foreground">
+                              {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? [])}
+                            </p>
+                          )}
                         </td>
                         <td className="px-4 py-3 w-28">
                         <input

Frontend/erp-system/app/dashboard/sales/invoices/[id]/print/page.tsx

@@ -7,6 +7,7 @@ import { ArrowLeft, Printer } from "lucide-react"
 import { salesApi } from "@/lib/api/sales"
 import { customersApi } from "@/lib/api/customers"
 import { itemsApi } from "@/lib/api/items"
+import { uomName } from "@/lib/uom"
 import { uomsApi } from "@/lib/api/uoms"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { errorMessage } from "@/lib/error-map"
@@ -143,7 +144,7 @@ export default function SalesInvoicePrintPage({ params }: { params: { id: string
                     <div className="font-medium text-foreground">{line.description}</div>
                     <div className="text-xs text-muted-foreground">SKU {items.find((i) => i.itemId === line.itemId)?.sku ?? `#${line.itemId}`}</div>
                   </TableCell>
-                  <TableCell>{uoms.find((u) => u.uomId === line.uomId)?.name ?? `#${line.uomId}`}</TableCell>
+                  <TableCell>{uomName(line.uomId, uoms)}</TableCell>
                   <TableCell className="text-right">{line.qty.toFixed(2)}</TableCell>
                   <TableCell className="text-right">{line.freeQty > 0 ? line.freeQty.toFixed(2) : "—"}</TableCell>
                   <TableCell className="text-right">{line.unitPrice.toFixed(2)}</TableCell>

Frontend/erp-system/app/dashboard/sales/invoices/new/page.tsx

@@ -12,6 +12,8 @@ import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from "@
 import { Badge } from "@/components/ui/badge"
 import { Table, TableBody, TableCell, TableHead, TableHeader, TableRow } from "@/components/ui/table"
 import { cn } from "@/lib/utils"
+import { basePreview, pickerOptions } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { getSuggestedUnitPrice } from "@/lib/sales-line-utils"
 import { errorMessage } from "@/lib/error-map"
 import { salesApi } from "@/lib/api/sales"
@@ -71,6 +73,7 @@ export default function NewSalesInvoicePage() {
   const [loading, setLoading] = useState(true)
   const [submitError, setSubmitError] = useState<string | null>(null)
   const [saving, setSaving] = useState(false)
+  const allowedUoms = useAllowedUoms()
 
   useEffect(() => {
     Promise.all([
@@ -129,6 +132,9 @@ export default function NewSalesInvoicePage() {
 
   function selectItem(key: string, itemId: number) {
     const item = items.find((candidate) => candidate.itemId === itemId)
+    // Load the units this item can actually be sold in, so the UOM picker below narrows from
+    // the global list to base + defined conversions.
+    allowedUoms.load(itemId)
     updateLine(key, {
       itemId,
       uomId: item?.baseUomId ?? 0,
@@ -363,7 +369,7 @@ export default function NewSalesInvoicePage() {
                           <SelectValue placeholder="UOM" />
                         </SelectTrigger>
                         <SelectContent>
-                          {uoms.map((u) => (
+                          {pickerOptions(allowedUoms.get(line.itemId), uoms).map((u) => (
                             <SelectItem key={u.uomId} value={String(u.uomId)}>
                               {u.name}
                             </SelectItem>
@@ -380,6 +386,12 @@ export default function NewSalesInvoicePage() {
                         onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) })}
                         className="h-9 w-24 text-right font-mono text-sm tabular-nums"
                       />
+                      {/* Makes the conversion visible while editing instead of at post time. */}
+                      {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? []) && (
+                        <p className="mt-1 text-xs text-muted-foreground">
+                          {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? [])}
+                        </p>
+                      )}
                     </TableCell>
                     <TableCell className="px-4 py-2">
                       <Input

Frontend/erp-system/app/dashboard/sales/slips/[id]/page.tsx

@@ -7,6 +7,8 @@ import { ArrowLeft, ExternalLink, Minus, Plus, Printer, Save, Send, X } from "lu
 import { salesApi } from "@/lib/api/sales"
 import { customersApi } from "@/lib/api/customers"
 import { itemsApi } from "@/lib/api/items"
+import { basePreview, pickerOptions, uomName } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { uomsApi } from "@/lib/api/uoms"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { usersApi } from "@/lib/api/users"
@@ -71,6 +73,7 @@ export default function SalesSlipDetailPage({ params }: { params: Promise<{ id:
   const [error, setError] = useState<string | null>(null)
   const [saving, setSaving] = useState(false)
   const [actionBusy, setActionBusy] = useState<"post" | "cancel" | null>(null)
+  const allowedUoms = useAllowedUoms()
 
   useEffect(() => {
     if (!Number.isFinite(slipId)) {
@@ -97,6 +100,7 @@ export default function SalesSlipDetailPage({ params }: { params: Promise<{ id:
         setWarehouseId(doc.data.warehouseId)
         setCashierUserId(doc.data.cashierUserId)
         setPromotionSuggestion(null)
+        doc.data.lines.forEach((line) => allowedUoms.load(line.itemId))
         setLines(
           doc.data.lines.map((line) => ({
             key: String(line.salesSlipLineId),
@@ -149,8 +153,12 @@ export default function SalesSlipDetailPage({ params }: { params: Promise<{ id:
   }
 
   function selectItem(key: string, itemId: number) {
+    allowedUoms.load(itemId)
     updateLine(key, {
       itemId,
+      // Reset to the new item's base unit: a UOM carried over from the previous item
+      // usually has no conversion for this one, and would be refused on save.
+      uomId: items.find((i) => i.itemId === itemId)?.baseUomId ?? 0,
       unitPrice: getSuggestedUnitPrice(items, itemId),
     })
   }
@@ -346,7 +354,7 @@ export default function SalesSlipDetailPage({ params }: { params: Promise<{ id:
                         : "No price suggestion available"}
                     </div>
                   </TableCell>
-                  <TableCell className="px-4 py-3">{uoms.find((u) => u.uomId === line.uomId)?.name ?? `#${line.uomId}`}</TableCell>
+                  <TableCell className="px-4 py-3">{uomName(line.uomId, uoms)}</TableCell>
                   <TableCell className="px-4 py-3 text-right">{line.qty.toFixed(0)}</TableCell>
                   <TableCell className="px-4 py-3 text-right">{line.freeQty > 0 ? line.freeQty.toFixed(0) : "-"}</TableCell>
                   <TableCell className="px-4 py-3 text-right">{money.format(line.unitPrice)}</TableCell>
@@ -457,10 +465,15 @@ export default function SalesSlipDetailPage({ params }: { params: Promise<{ id:
                     <TableCell>
                       <Select value={line.uomId ? String(line.uomId) : ""} onValueChange={(v) => updateLine(line.key, { uomId: Number(v) })} disabled={locked}>
                         <SelectTrigger className="h-11!"><SelectValue placeholder="UOM" /></SelectTrigger>
-                        <SelectContent>{uoms.map((u) => <SelectItem key={u.uomId} value={String(u.uomId)}>{u.name}</SelectItem>)}</SelectContent>
+                        <SelectContent>{pickerOptions(allowedUoms.get(line.itemId), uoms).map((u) => <SelectItem key={u.uomId} value={String(u.uomId)}>{u.name}</SelectItem>)}</SelectContent>
                       </Select>
                     </TableCell>
-                    <TableCell><Input type="number" min="0" step="0.01" value={line.qty} onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) })} disabled={locked} /></TableCell>
+                    <TableCell>
+                      <Input type="number" min="0" step="0.01" value={line.qty} onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) })} disabled={locked} />
+                      {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? []) && (
+                        <p className="mt-1 text-xs text-muted-foreground">{basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? [])}</p>
+                      )}
+                    </TableCell>
                     <TableCell><Input type="number" min="0" step="0.01" value={line.freeQty} onChange={(e) => updateLine(line.key, { freeQty: Number(e.target.value) })} disabled={locked} /></TableCell>
                     <TableCell><Input type="number" min="0" step="0.01" value={line.unitPrice ?? ""} onChange={(e) => updateLine(line.key, { unitPrice: e.target.value === "" ? null : Number(e.target.value) })} disabled={locked} /></TableCell>
                     <TableCell><Button type="button" variant="ghost" size="icon" onClick={() => removeLine(line.key)} disabled={locked}><Minus className="size-4" /></Button></TableCell>

Frontend/erp-system/app/dashboard/sales/slips/new/page.tsx

@@ -12,6 +12,8 @@ import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from "@
 import { Badge } from "@/components/ui/badge"
 import { Table, TableBody, TableCell, TableHead, TableHeader, TableRow } from "@/components/ui/table"
 import { cn } from "@/lib/utils"
+import { basePreview, pickerOptions } from "@/lib/uom"
+import { useAllowedUoms } from "@/hooks/use-allowed-uoms"
 import { getSuggestedUnitPrice } from "@/lib/sales-line-utils"
 import { errorMessage } from "@/lib/error-map"
 import { salesApi } from "@/lib/api/sales"
@@ -65,6 +67,7 @@ export default function NewSalesSlipPage() {
   const [lines, setLines] = useState<Line[]>([blankLine("line-1")])
   const [loading, setLoading] = useState(true)
   const [submitError, setSubmitError] = useState<string | null>(null)
+  const allowedUoms = useAllowedUoms()
   const [saving, setSaving] = useState(false)
 
   useEffect(() => {
@@ -104,6 +107,8 @@ export default function NewSalesSlipPage() {
 
   function selectItem(key: string, itemId: number) {
     const item = items.find((candidate) => candidate.itemId === itemId)
+    // Narrow the UOM picker to units this item has a conversion for.
+    allowedUoms.load(itemId)
     updateLine(key, {
       itemId,
       uomId: item?.baseUomId ?? 0,
@@ -334,7 +339,7 @@ export default function NewSalesSlipPage() {
                           <SelectValue placeholder="UOM" />
                         </SelectTrigger>
                         <SelectContent>
-                          {uoms.map((u) => (
+                          {pickerOptions(allowedUoms.get(line.itemId), uoms).map((u) => (
                             <SelectItem key={u.uomId} value={String(u.uomId)}>
                               {u.name}
                             </SelectItem>
@@ -351,6 +356,12 @@ export default function NewSalesSlipPage() {
                         onChange={(e) => updateLine(line.key, { qty: Number(e.target.value) })}
                         className="h-9 w-24 text-right font-mono text-sm tabular-nums"
                       />
+                      {/* Makes the conversion visible while editing instead of at post time. */}
+                      {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? []) && (
+                        <p className="mt-1 text-xs text-muted-foreground">
+                          {basePreview(line.qty, line.uomId, allowedUoms.get(line.itemId) ?? [])}
+                        </p>
+                      )}
                     </TableCell>
                     <TableCell className="px-4 py-2">
                       <Input

Frontend/erp-system/app/dashboard/stock/enquiry/page.tsx

@@ -7,6 +7,7 @@ import { PackageSearch, Search } from "lucide-react"
 import { stockApi } from "@/lib/api/stock"
 import { itemsApi } from "@/lib/api/items"
 import { warehousesApi } from "@/lib/api/warehouses"
+import { formatQtyValue, formatQtyWithName } from "@/lib/uom"
 import { errorMessage } from "@/lib/error-map"
 import { OnHand } from "@/types/stock"
 import { ItemListItem, Warehouse } from "@/types/master-data"
@@ -135,11 +136,12 @@ export default function StockEnquiryPage() {
                     <div className="text-sm text-muted-foreground">{item?.name}</div>
                   </TableCell>
                   <TableCell className="px-3 py-3.5">{wh?.code ?? `#${row.warehouseId}`}</TableCell>
-                  <TableCell className="px-3 py-3.5">{row.onHand}</TableCell>
+                  {/* Every figure here is base UOM; the label is what tells the user which. */}
-                  <TableCell className="px-3 py-3.5 font-medium">{row.available}</TableCell>
+                  <TableCell className="px-3 py-3.5">{formatQtyWithName(row.onHand, row.baseUomName)}</TableCell>
-                  <TableCell className="px-3 py-3.5">{row.onHold}</TableCell>
+                  <TableCell className="px-3 py-3.5 font-medium">{formatQtyWithName(row.available, row.baseUomName)}</TableCell>
-                  <TableCell className="px-3 py-3.5">{row.inTransit}</TableCell>
+                  <TableCell className="px-3 py-3.5">{formatQtyValue(row.onHold)}</TableCell>
-                  <TableCell className="px-3 py-3.5">{row.reserved}</TableCell>
+                  <TableCell className="px-3 py-3.5">{formatQtyValue(row.inTransit)}</TableCell>
+                  <TableCell className="px-3 py-3.5">{formatQtyValue(row.reserved)}</TableCell>
                   <TableCell className="px-3 py-3.5">
                     <Link
                       href={`/dashboard/stock/valuation?itemId=${row.itemId}&warehouseId=${row.warehouseId}`}

Frontend/erp-system/app/dashboard/stock/ledger/page.tsx

@@ -6,6 +6,7 @@ import { ChevronLeft, ChevronRight, ScrollText } from "lucide-react"
 import { stockApi } from "@/lib/api/stock"
 import { itemsApi } from "@/lib/api/items"
 import { warehousesApi } from "@/lib/api/warehouses"
+import { formatQtyWithName } from "@/lib/uom"
 import { errorMessage } from "@/lib/error-map"
 import { cn } from "@/lib/utils"
 import { LedgerEntry } from "@/types/stock"
@@ -189,10 +190,10 @@ export default function StockLedgerPage() {
                         {entry.direction}
                       </Badge>
                     </TableCell>
-                    <TableCell className="px-3 py-3.5">{entry.qtyBase}</TableCell>
+                    <TableCell className="px-3 py-3.5">{formatQtyWithName(entry.qtyBase, entry.baseUomName)}</TableCell>
                     <TableCell className="px-3 py-3.5">{entry.unitCost.toFixed(2)}</TableCell>
                     <TableCell className="px-3 py-3.5">{entry.value.toFixed(2)}</TableCell>
-                    <TableCell className="px-3 py-3.5 font-medium">{entry.runningBalance}</TableCell>
+                    <TableCell className="px-3 py-3.5 font-medium">{formatQtyWithName(entry.runningBalance, entry.baseUomName)}</TableCell>
                     <TableCell className="px-3 py-3.5 text-muted-foreground">
                       {entry.sourceDocType} #{entry.sourceDocId}
                     </TableCell>

Frontend/erp-system/app/dashboard/stock/reorder-alerts/page.tsx

@@ -6,9 +6,11 @@ import { AlertTriangle, CheckCircle2 } from "lucide-react"
 import { stockApi } from "@/lib/api/stock"
 import { itemsApi } from "@/lib/api/items"
 import { warehousesApi } from "@/lib/api/warehouses"
+import { uomsApi } from "@/lib/api/uoms"
 import { errorMessage } from "@/lib/error-map"
+import { formatQty } from "@/lib/uom"
 import { ReorderAlert } from "@/types/stock"
-import { ItemListItem, Warehouse } from "@/types/master-data"
+import { ItemListItem, Uom, Warehouse } from "@/types/master-data"
 
 import { Button } from "@/components/ui/button"
 import { Skeleton } from "@/components/ui/skeleton"
@@ -19,16 +21,23 @@ export default function ReorderAlertsPage() {
   const [alerts, setAlerts] = useState<ReorderAlert[] | null>(null)
   const [items, setItems] = useState<ItemListItem[] | null>(null)
   const [warehouses, setWarehouses] = useState<Warehouse[] | null>(null)
+  const [uoms, setUoms] = useState<Uom[]>([])
   const [error, setError] = useState<string | null>(null)
   const [requesting, setRequesting] = useState<string | null>(null)
   const [requested, setRequested] = useState<Set<string>>(new Set())
 
   useEffect(() => {
-    Promise.all([stockApi.reorderAlerts(), itemsApi.list({ pageSize: 200 }), warehousesApi.list()])
+    Promise.all([
-      .then(([a, it, wh]) => {
+      stockApi.reorderAlerts(),
+      itemsApi.list({ pageSize: 200 }),
+      warehousesApi.list(),
+      uomsApi.list({ pageSize: 200 }),
+    ])
+      .then(([a, it, wh, uo]) => {
         setAlerts(a.items)
         setItems(it.items)
         setWarehouses(wh.items)
+        setUoms(uo.items)
       })
       .catch((err) => setError(errorMessage(err)))
   }, [])
@@ -44,7 +53,10 @@ export default function ReorderAlertsPage() {
       const res = await stockApi.createReorderRequisition(alert.itemId, alert.warehouseId)
       setRequested((prev) => new Set(prev).add(key))
       const qty = res.lines[0]?.qty ?? alert.suggestedRequisitionQty
-      toast.success("Requisition created", `${res.docNo} for ${qty} units.`)
+      // Requisitions are base-UOM only, so name the item's actual base unit rather than
+      // the placeholder word "units".
+      const uom = itemsById.get(alert.itemId)?.baseUomId
+      toast.success("Requisition created", `${res.docNo} for ${formatQty(qty, uom ?? null, uoms)}.`)
     } catch (err) {
       toast.error("Could not create requisition", errorMessage(err))
     } finally {

Frontend/erp-system/app/print/sales/invoices/[id]/page.tsx

@@ -6,6 +6,7 @@ import { Printer } from "lucide-react"
 import { salesApi } from "@/lib/api/sales"
 import { customersApi } from "@/lib/api/customers"
 import { itemsApi } from "@/lib/api/items"
+import { uomName } from "@/lib/uom"
 import { uomsApi } from "@/lib/api/uoms"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { errorMessage } from "@/lib/error-map"
@@ -131,7 +132,7 @@ export default function SalesInvoicePrintPage({ params }: { params: Promise<{ id
                   <div className="font-medium text-foreground">{line.description}</div>
                   <div className="text-xs text-muted-foreground">SKU {items.find((i) => i.itemId === line.itemId)?.sku ?? `#${line.itemId}`}</div>
                 </TableCell>
-                <TableCell>{uoms.find((u) => u.uomId === line.uomId)?.name ?? `#${line.uomId}`}</TableCell>
+                <TableCell>{uomName(line.uomId, uoms)}</TableCell>
                 <TableCell className="text-right">{line.qty.toFixed(2)}</TableCell>
                 <TableCell className="text-right">{line.freeQty > 0 ? line.freeQty.toFixed(2) : "—"}</TableCell>
                 <TableCell className="text-right">{line.unitPrice.toFixed(2)}</TableCell>

Frontend/erp-system/app/print/sales/slips/[id]/page.tsx

@@ -6,6 +6,7 @@ import { Printer } from "lucide-react"
 import { salesApi } from "@/lib/api/sales"
 import { customersApi } from "@/lib/api/customers"
 import { itemsApi } from "@/lib/api/items"
+import { uomName } from "@/lib/uom"
 import { uomsApi } from "@/lib/api/uoms"
 import { warehousesApi } from "@/lib/api/warehouses"
 import { usersApi } from "@/lib/api/users"
@@ -123,7 +124,7 @@ export default function SalesSlipPrintPage({ params }: { params: Promise<{ id: s
                   <div className="font-medium text-foreground">{line.description}</div>
                   <div className="text-xs text-muted-foreground">SKU {items.find((i) => i.itemId === line.itemId)?.sku ?? `#${line.itemId}`}</div>
                 </TableCell>
-                <TableCell>{uoms.find((u) => u.uomId === line.uomId)?.name ?? `#${line.uomId}`}</TableCell>
+                <TableCell>{uomName(line.uomId, uoms)}</TableCell>
                 <TableCell className="text-right">{line.qty.toFixed(0)}</TableCell>
                 <TableCell className="text-right">{line.freeQty > 0 ? line.freeQty.toFixed(0) : "—"}</TableCell>
                 <TableCell className="text-right">{line.unitPrice.toFixed(2)}</TableCell>

Frontend/erp-system/hooks/use-allowed-uoms.ts

@@ -0,0 +1,73 @@
+"use client"
+
+// Per-item allowed-UOM lookup for document line forms.
+//
+// Every line on a document can reference a different item, so this caches by itemId and
+// fetches each one once. A naive refetch on every line edit would cost one request per line
+// on a 20-line invoice; the cache keeps it to one request per distinct item.
+import { useCallback, useEffect, useRef, useState } from "react"
+
+import { itemsApi } from "@/lib/api/items"
+import { AllowedUom } from "@/types/master-data"
+
+export interface AllowedUomLookup {
+  /** Allowed units for an item, or `undefined` while it is still loading / unknown. */
+  get: (itemId: number | null | undefined) => AllowedUom[] | undefined
+  /** Ensure an item's units are loaded. Safe to call repeatedly; in-flight requests are shared. */
+  load: (itemId: number | null | undefined) => void
+}
+
+export function useAllowedUoms(): AllowedUomLookup {
+  const [cache, setCache] = useState<Record<number, AllowedUom[]>>({})
+  // Tracks in-flight and failed ids so a repeated render never re-issues the same request.
+  const pending = useRef<Set<number>>(new Set())
+
+  const load = useCallback((itemId: number | null | undefined) => {
+    if (!itemId || pending.current.has(itemId)) return
+    pending.current.add(itemId)
+    itemsApi
+      .allowedUoms(itemId)
+      .then((uoms) => setCache((prev) => ({ ...prev, [itemId]: uoms })))
+      .catch(() => {
+        // Leave the id marked so we don't hammer a failing endpoint. The picker falls back
+        // to the global UOM list (see `pickerOptions`), and the server still rejects an
+        // invalid unit on save — this is a degraded experience, not a correctness hole.
+      })
+  }, [])
+
+  const get = useCallback(
+    (itemId: number | null | undefined) => (itemId ? cache[itemId] : undefined),
+    [cache],
+  )
+
+  return { get, load }
+}
+
+/**
+ * Single-item variant for screens with one item in scope (the conversion editor, a stock
+ * enquiry filtered to one item).
+ */
+export function useItemAllowedUoms(itemId: number | null | undefined): AllowedUom[] {
+  const [uoms, setUoms] = useState<AllowedUom[]>([])
+
+  useEffect(() => {
+    if (!itemId) {
+      setUoms([])
+      return
+    }
+    let cancelled = false
+    itemsApi
+      .allowedUoms(itemId)
+      .then((res) => {
+        if (!cancelled) setUoms(res)
+      })
+      .catch(() => {
+        if (!cancelled) setUoms([])
+      })
+    return () => {
+      cancelled = true
+    }
+  }, [itemId])
+
+  return uoms
+}

Frontend/erp-system/lib/api/items.ts

@@ -3,6 +3,7 @@
 import { apiRequest, apiRequestWithETag, buildQuery } from "@/lib/api-client"
 import { ApiResult, EntityStatus, PagedResponse } from "@/types/common"
 import {
+  AllowedUom,
   CreateItemRequest,
   Item,
   ItemListItem,
@@ -60,10 +61,20 @@ export const itemsApi = {
     })
   },
 
+  /**
+   * Replace the item's conversions. Every row must be `<other> → baseUomId` with a factor
+   * greater than zero; the reverse direction, a self-conversion, or a row from the base UOM
+   * is rejected with 422 (the engine only ever looks up `<other> → base` and never inverts).
+   */
   updateUomConversions(itemId: number, request: UpdateUomConversionsRequest): Promise<UpdateUomConversionsResponse> {
     return apiRequest<UpdateUomConversionsResponse>(`/items/${itemId}/uom-conversions`, {
       method: "PUT",
       body: request,
     })
   },
+
+  /** The UOMs this item can be transacted in — base UOM first, then each conversion source. */
+  allowedUoms(itemId: number): Promise<AllowedUom[]> {
+    return apiRequest<AllowedUom[]>(`/items/${itemId}/uoms`)
+  },
 }

Frontend/erp-system/lib/uom.ts

@@ -0,0 +1,94 @@
+// The single place the UI resolves and renders units of measure.
+//
+// Before this module, ~10 screens each re-declared their own inline `uoms.find(...)` under
+// three different names, with two different unknown-UOM fallbacks (`#12` vs `UOM 12`), and
+// there was no quantity formatter anywhere. Everything unit-shaped goes through here now.
+//
+// The one rule these helpers encode: **conversion toward base is authoritative.** The server
+// stores stock, layers and the ledger exclusively in an item's base UOM, and resolves the
+// base quantity when a document line is saved. `toBase` below exists to *preview* that for
+// the user at entry time; it is never the source of what gets posted.
+import { AllowedUom, Uom } from "@/types/master-data"
+
+/** Shown when a uomId has no matching row — one fallback across the whole app. */
+const UNKNOWN_UOM = "—"
+
+/** Quantities are `decimal(18,4)` server-side; trailing zeros are noise in a table. */
+const QTY_FORMATTER = new Intl.NumberFormat("en-US", {
+  minimumFractionDigits: 0,
+  maximumFractionDigits: 4,
+})
+
+/** `12` -> "PCS". Accepts any list with `uomId`/`name`, so `Uom[]` and `AllowedUom[]` both work. */
+export function uomName(
+  uomId: number | null | undefined,
+  uoms: readonly { uomId: number; name: string }[],
+): string {
+  if (uomId === null || uomId === undefined || uomId === 0) return UNKNOWN_UOM
+  return uoms.find((u) => u.uomId === uomId)?.name ?? UNKNOWN_UOM
+}
+
+/** `1234.5` -> "1,234.5". Quantity-specific: unlike `formatAmount` it does not force 2dp. */
+export function formatQtyValue(qty: number | null | undefined): string {
+  if (qty === null || qty === undefined || Number.isNaN(qty)) return "—"
+  return QTY_FORMATTER.format(qty)
+}
+
+/**
+ * `(24, 12, uoms)` -> "24 PCS". The formatter every screen showing a quantity should use —
+ * a bare number leaves the user guessing which unit a figure is in.
+ */
+export function formatQty(
+  qty: number | null | undefined,
+  uomId: number | null | undefined,
+  uoms: readonly { uomId: number; name: string }[],
+): string {
+  const value = formatQtyValue(qty)
+  const unit = uomName(uomId, uoms)
+  return unit === UNKNOWN_UOM ? value : `${value} ${unit}`
+}
+
+/** Convenience for stock screens, whose DTOs carry `baseUomName` directly from the server. */
+export function formatQtyWithName(qty: number | null | undefined, uomName: string | null | undefined): string {
+  const value = formatQtyValue(qty)
+  return uomName ? `${value} ${uomName}` : value
+}
+
+/**
+ * Converts an entered quantity to the item's base UOM, matching the server's arithmetic
+ * (multiply by the factor, round to 4dp). Display only — the authoritative base quantity is
+ * the one the server resolves and snapshots when the line is saved.
+ */
+export function toBase(qty: number, factor: number): number {
+  if (!Number.isFinite(qty) || !Number.isFinite(factor)) return 0
+  return Math.round(qty * factor * 10_000) / 10_000
+}
+
+/**
+ * The hint rendered beside a quantity input: `"= 24 PCS"` when the chosen unit is not the
+ * item's base, and `null` when it is (a "= 24 PCS" next to "24 PCS" is just noise).
+ *
+ * Making the conversion visible at entry is the point — previously a user only discovered a
+ * unit mismatch as a 422 when they tried to post the finished document.
+ */
+export function basePreview(
+  qty: number | null | undefined,
+  uomId: number | null | undefined,
+  allowed: readonly AllowedUom[],
+): string | null {
+  if (!qty || !uomId) return null
+  const selected = allowed.find((u) => u.uomId === uomId)
+  const base = allowed.find((u) => u.isBase)
+  if (!selected || !base || selected.isBase) return null
+  return `= ${formatQtyValue(toBase(qty, selected.factor))} ${base.name}`
+}
+
+/**
+ * Falls back to the global UOM list while an item's allowed units are still loading (or when
+ * no item is chosen yet), so a picker never renders empty. Once `allowed` arrives it wins —
+ * that narrowing is the whole reason the endpoint exists.
+ */
+export function pickerOptions(allowed: readonly AllowedUom[] | undefined, all: readonly Uom[]): { uomId: number; name: string }[] {
+  if (allowed && allowed.length > 0) return allowed.map((u) => ({ uomId: u.uomId, name: u.name }))
+  return all.map((u) => ({ uomId: u.uomId, name: u.name }))
+}

Frontend/erp-system/types/master-data.ts

@@ -33,6 +33,11 @@ export interface ItemReorderSetting {
   reorderQty: number
 }
 
+/**
+ * A per-item conversion row. Always directional: `fromUom → toUom`, where `toUom` is
+ * required by the server to be the item's `baseUomId`. Quantity in `fromUom` × `factor`
+ * = quantity in base UOM. Saving one in the opposite direction is rejected with 422.
+ */
 export interface UomConversion {
   conversionId: number
   fromUom: number
@@ -40,6 +45,20 @@ export interface UomConversion {
   factor: number
 }
 
+/**
+ * A UOM an item may actually be transacted in — from `GET /items/{itemId}/uoms`.
+ *
+ * Line forms must populate their UOM picker from this rather than the global `uoms` list:
+ * choosing a unit the item has no conversion for is rejected when the line is saved.
+ */
+export interface AllowedUom {
+  uomId: number
+  name: string
+  /** Multiply a quantity in this UOM by `factor` to get the item's base UOM. 1 for the base itself. */
+  factor: number
+  isBase: boolean
+}
+
 /**
  * Full Item resource (docs/11 §2.1 `GET /items/{itemId}`).
  *

Frontend/erp-system/types/procurement.ts

@@ -138,11 +138,23 @@ export interface PoLine {
   itemId: number
   uomId: number
   warehouseId: number
+  /** Ordered quantity, in `uomId`. */
   qty: number
   unitPrice: number
   /** Rate, 0..1 (e.g. 0.18), not an amount. */
   tax: number
+  /**
+   * Received to date, in `uomId`. **Display only** — the server derives it by dividing
+   * `qtyReceivedBase` by `conversionFactor`, so it can drift. Use the base pair below for
+   * any remaining/outstanding/closed logic.
+   */
   qtyReceived: number
+  /** Ordered quantity in the item's base UOM — what over-receipt and PO closure compare on. */
+  qtyBase: number
+  /** Received to date in the item's base UOM. Authoritative. */
+  qtyReceivedBase: number
+  /** Factor used to derive the base pair from `uomId`; 1 when the line is in base UOM. */
+  conversionFactor: number
 }
 
 export interface PoTotals {

Frontend/erp-system/types/stock.ts

@@ -19,6 +19,9 @@ export interface OnHand {
   /** Always 0 in Phase 1 (sales reservation stub, FR-STK-11). */
   reserved: number
   asOf: string
+  /** The item's base UOM — every quantity above is in it. For labelling, not conversion. */
+  baseUomId: number
+  baseUomName: string
 }
 
 // --- 5.2 Ledger -------------------------------------------------------------------
@@ -41,6 +44,9 @@ export interface LedgerEntry {
   sourceDocId: number
   userId: number
   createdAt: string
+  /** The item's base UOM — `qtyBase` and `runningBalance` are in it. */
+  baseUomId: number
+  baseUomName: string
 }
 
 // --- 5.3 Valuation ------------------------------------------------------------------