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,
-                UomId = item.BaseUomId,
+                Qty = r.Qty,
+                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;
-                if (input.Qty > openQty * (1 + OverReceiptTolerance))
+                // Compare in base UOM: a GRN may legitimately receive in a different UOM from
+                // 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).
-                var (qtyBase, unitCostBase) = await ToBaseAsync(item, line.UomId, line.Qty, line.NetUnitCost, token);
+                // enters stock value (docs/10 FR-GRN-06, revised). Quantity and cost come from
+                // 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)
-            po.Lines.Add(ToLine(input));
+        foreach (var line in await ToLinesAsync(request.Lines, ct))
+            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,
-        UomId = l.UomId,
-        WarehouseId = l.WarehouseId,
-        Qty = l.Qty,
-        UnitPrice = l.UnitPrice,
-        Tax = l.Tax,
-        QtyReceived = 0
-    };
+        var lines = new List<PoLine>(inputs.Count);
+        foreach (var l in inputs)
+        {
+            var item = await _items.Query().AsNoTracking().FirstAsync(i => i.ItemId == l.ItemId, ct);
+            var factor = await _uomConverter.ResolveFactorAsync(item, l.UomId, ct);
+            lines.Add(new PoLine
+            {
+                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
-            // 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)),
+            getLines: x => x.Lines.Where(l => l.IncludeInBundle).Select(l => new PostingLine(l.ItemId, l.WarehouseId, l.QtyBase, l.QtyBase, 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)
-        => _fifo.GetValuationAsync(itemId, warehouseId, ct);
+    public async Task<StockValuationDto> GetValuationAsync(int itemId, int warehouseId, CancellationToken ct = default)
+    {
+        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.
-        return (qty * conv.Factor, unitCostPerUom / conv.Factor);
+        // total value is preserved. Round to each column's own scale here rather than
+        // 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);
+    }
 }