ERP_Distribution/Distribution/docs/15_procurement.md

# 15 — Procurement / Purchasing

**Status**: Done — migration applied to the database
**Depends On**: 01 — Item Catalog (Done), 14 — Pricing & Supplier Master Data (Done)

---

## Goal

Covers [mainPlan.md §9](./mainPlan.md#9-procurement--purchasing) — inbound stock from suppliers,
the mirror of Order Management's outbound flow to stores. Purchase Requisition is deliberately
**not** part of this scope (see Background/Context) — `createPurchaseOrder` is the entry point.

---

## Background / Context

Inbound stock (`createGoodsReceipt` in [04_loading_event.md](./04_loading_event.md)/
[mainPlan.md §6](./mainPlan.md#6-warehouse-management)) had no purchase order to receive against —
this step creates that PO in the first place.

Sales reps take orders **from** shops (existing `Features/Order`, outbound — decreases stock,
references `Store`, prices via `PriceListItem.Price`); procurement is the mirror — bringing stock
**in** from suppliers (`Features/PurchaseOrder`, inbound — increases stock, references `Supplier`,
costs via `PurchaseOrderItem.UnitCost`). Same header+line-item shape, opposite direction and
counterparty, which is why they're separate features rather than one generic "Order" entity.

### No requisition step — how "raise a request to purchase" is actually handled

[mainPlan.md §9](./mainPlan.md#9-procurement--purchasing) originally listed
`createPurchaseRequisition(items[], requestedBy)` as a candidate function. **Dropped from this
app's design** — the request and the decision to buy collapse into one step:

- Whoever notices stock is low (owner, manager, or eventually an automated low-stock notice) calls
  `createPurchaseOrder(supplierId, items[])` directly. Issuing the PO **is** the request; it needs
  no separate approval because only an already-authenticated user can call it (see Permissions —
  enforcement itself is deferred, matching Supplier/PriceList).
- There is no Draft → Submitted → Approved chain because restocking authority isn't delegated down
  to sales reps the way order-taking is.
- A lower-level "we're low on X" signal is `sendLowStockAlert` (§15 Notifications & Alerts, not yet
  built) — a notification, not a requisition with its own approval workflow.

### Connections to existing modules

- **Item Catalog** ([01_item_catalog.md](./01_item_catalog.md)): `PurchaseOrderItem.FK_ItemId`,
  `GoodsReceiptItem` (via `PurchaseOrderItemId` → `PurchaseOrderItem.FK_ItemId`), and
  `PurchaseReturnItem.FK_ItemId` all reference the same `Item` table sales orders use — one
  catalog, two directions of stock movement.
- **Stock Ledger** ([02_stock_ledger.md](./02_stock_ledger.md)): procurement writes to the same
  `StockBalance`/`StockMovement` tables as the rest of the system rather than a separate stock
  field — `createGoodsReceipt` credits `StockBalance` (`MovementType.PurchaseReceipt`),
  `createPurchaseReturn` debits it (`MovementType.PurchaseReturn`). No new stock model was needed.
- **Pricing** ([14_pricing_and_suppliers.md](./14_pricing_and_suppliers.md)): `PriceListItem.Price`
  (what you charge a shop) and `PurchaseOrderItem.UnitCost` (what you pay a supplier) are
  deliberately separate, unlinked fields — procurement never reads or writes `PriceList` rows.
  Cost-vs-sale margin reporting is left for a later reporting step (see Open Questions).
- **Supplier Master Data** ([14_pricing_and_suppliers.md](./14_pricing_and_suppliers.md)):
  `PurchaseOrder.FK_SupplierId` links every PO to one supplier. `SupplierService.EvaluateAsync`
  (`evaluateSupplier`) is now the only writer of `Supplier.PerformanceScore`, and
  `SupplierService.DeleteAsync` now blocks soft-delete if any active `PurchaseOrder` references the
  supplier — both were previously open items in 14's doc, now closed by this step.
- **Order Management** ([mainPlan.md §1](./mainPlan.md#1-order-management)): while implementing
  this step, `OrderService.CreateAsync` was found to have no `checkStockAvailability` check at all
  (mainPlan's own documented gap) — fixed alongside this step: it now rejects an order if requested
  quantity exceeds total on-hand `StockBalance` for the item, summed across all locations. It still
  does not decrement stock at order time — that stays deferred to loading/delivery (§04-06), per the
  existing architecture, to avoid double-decrementing once those steps are built.

---

## Data Model Changes

Six new tables — three header/line-item pairs (mirrors the `PriceList`/`PriceListItem` split).

### PurchaseOrder
```
PurchaseOrder
├── Id (Guid)                  — PK, from Template
├── FK_SupplierId (Guid)       — required
├── FK_CreatedByUserId (Guid)  — required, the user who issued the PO
├── Status (enum)              — Issued, PartiallyReceived, Received, Closed, Cancelled
├── CancelReason (string?)     — set by cancelPurchaseOrder
├── IsActive, CreatedAt        — from Template
```

### PurchaseOrderItem
```
PurchaseOrderItem
├── Id (Guid)                  — PK, from Template
├── FK_PurchaseOrderId (Guid)  — required
├── FK_ItemId (Guid)           — required
├── Quantity (int)             — ordered quantity
├── UnitCost (decimal)         — agreed cost per unit at the time the PO was issued
├── ReceivedQuantity (int)     — running total, incremented by createGoodsReceipt; drives PO Status
├── CancelledQuantity (int)    — set by cancelPurchaseOrderLine, default 0; outstanding qty for
│                                 receiving = Quantity - ReceivedQuantity - CancelledQuantity
├── IsCancelled (bool)         — true once this line is individually cancelled, default false
```

> **Update (GRN rework):** a goods receipt no longer requires a PO — see "Direct goods receipt" and
> the costing / free-issue / tax additions in
> [refactor_docs/grn_costing_and_taxes.md](./refactor_docs/grn_costing_and_taxes.md). The tree
> below shows the original design plus those changes, marked *(later)*.

### GoodsReceipt
```
GoodsReceipt
├── Id (Guid)                  — PK, from Template
├── FK_PurchaseOrderId (Guid?) — nullable *(later)*: null = direct receipt (no PO)
├── FK_SupplierId (Guid?)      — *(later)* set on a direct receipt; a PO receipt reaches the supplier via the PO
├── DiscountPercent, VatPercent, NbtPercent (decimal 5,2) — *(later)* GRN-wide defaults for lines
├── FK_ReceivedByUserId (Guid) — required
├── FK_WarehouseId (Guid)      — required; the warehouse the accepted quantities are received into.
│                                 Not anticipated in the original design — added because
│                                 StockBalance/StockMovement are keyed by (LocationId, LocationType),
│                                 so a receiving location is mandatory to credit stock at all.
├── ReceivedAt (DateTime)
├── IsActive, CreatedAt        — from Template
```

### GoodsReceiptItem
```
GoodsReceiptItem
├── Id (Guid)                       — PK, from Template
├── FK_GoodsReceiptId (Guid)        — required
├── FK_PurchaseOrderItemId (Guid?)  — ties the receipt line back to its ordered line; null on a direct receipt
├── FK_ItemId (Guid?)               — *(later)* the item, on a direct receipt line
├── AcceptedQuantity (int)          — added to warehouse stock
├── RejectedQuantity (int)          — damaged/short at receipt, default 0, not added to stock
├── FreeQuantity (int)              — *(later)* free-issue units: added to stock, not costed, not counted against the PO
├── UnitCost (decimal 18,2)         — *(later)* cost per unit; PO receipts default to the PO line's UnitCost
├── DiscountPercent, NbtPercent, VatPercent (decimal 5,2) — *(later)* per-line rates
```

### PurchaseReturn

> **Not the same as `SaleReturn`** ([18_returns_claims.md](./18_returns_claims.md)), despite the
> similar name. `PurchaseReturn` moves goods **Distributor → Supplier** (debits the receiving
> warehouse); `SaleReturn` moves goods **Shop → Distributor** (credits a delivery vehicle). Also
> distinct from `SupplierClaim` ([18_returns_claims.md](./18_returns_claims.md)), which is for
> supplier disputes that don't involve physically returning goods (short-shipment, price dispute)
> and — unlike this entity — requires approval. See that doc's "Four distinct entities" table for
> the full comparison.

```
PurchaseReturn
├── Id (Guid)                  — PK, from Template
├── FK_PurchaseOrderId (Guid)  — required
├── Reason (string)
├── IsActive, CreatedAt        — from Template
```

### PurchaseReturnItem
```
PurchaseReturnItem
├── Id (Guid)                    — PK, from Template
├── FK_PurchaseReturnId (Guid)   — required
├── FK_ItemId (Guid)             — required
├── Quantity (int)               — returned quantity, decremented from warehouse stock
```
Stock is decremented at the warehouse of the PO's most recent `GoodsReceipt` (the current model
gives one receiving warehouse per PO in practice; see Open Questions for the multi-warehouse case).

---

## Migrations

1. **AddProcurement** (`20260824110925_AddProcurement`) — created `PurchaseOrders`,
   `PurchaseOrderItems`, `GoodsReceipts`, `GoodsReceiptItems`, `PurchaseReturns`,
   `PurchaseReturnItems`, with FKs to `Suppliers`/`Items`/`Warehouses` and indexes on every FK
   column. Applied via `dotnet ef database update`.
2. **AddDirectGoodsReceipt** (`20261006070042`) — made the PO links nullable and added the supplier /
   item links for direct receipts.
3. **AddGrnPricing**, **AddGrnFreeQuantity**, **AddGrnNbt** (`20261007…`, `20261008…`) — GRN costing,
   free issues and NBT. Pricing and free quantity are applied; **AddGrnNbt was still pending** when
   last checked, and GRN reads/writes fail until it is applied — see
   [refactor_docs/grn_costing_and_taxes.md](./refactor_docs/grn_costing_and_taxes.md) §7.

---

## API Endpoints

> No permission policy is enforced on any route below — matches the Supplier/PriceList convention
> from step 14 (bare `[Authorize]`, authentication required, no per-action policy). See
> Permissions to Add to Seed.

### Purchase Orders (`Features/PurchaseOrder`)
| Function reference | Endpoint |
|---|---|
| `createPurchaseOrder(supplierId, items[])` | `POST /api/purchase-orders` |
| `getPurchaseOrder(poId)` | `GET /api/purchase-orders/{id}` |
| `listPurchaseOrders(filters)` | `GET /api/purchase-orders` |
| `updatePurchaseOrder(poId, items[])` | `PUT /api/purchase-orders/{id}` |
| `closePurchaseOrder(poId)` | `POST /api/purchase-orders/{id}/close` |
| `cancelPurchaseOrder(poId, reason)` | `POST /api/purchase-orders/{id}/cancel` |
| `cancelPurchaseOrderLine(poId, poItemId, reason)` | `POST /api/purchase-orders/{id}/items/{itemId}/cancel` |
| `matchInvoiceToPO(poId, invoiceId?, grnId?)` | `POST /api/purchase-orders/{id}/match-invoice` |
| `resolveInvoiceVariance(poId, variance, resolution)` | `POST /api/purchase-orders/{id}/resolve-variance` |

### Goods Receipt (`Features/Grn`)
> A PO is **not mandatory** for a goods receipt: `POST /api/goods-receipts/direct` receives stock
> with no PO (no ordered-quantity limit, no PO status, no purchase returns / invoice matching for
> it). See [refactor_docs/grn_costing_and_taxes.md](./refactor_docs/grn_costing_and_taxes.md) §1 for
> when to use which.
| Function reference | Endpoint |
|---|---|
| `createGoodsReceipt(poId, warehouseId, items[])` | `POST /api/purchase-orders/{id}/goods-receipts` (also `POST /api/goods-receipts` with `purchaseOrderId` in the body) |
| `createDirectGoodsReceipt(warehouseId, supplierId?, items[])` | `POST /api/goods-receipts/direct` |
| `getGoodsReceipt(grnId)` | `GET /api/goods-receipts/{id}` |
| `listGoodsReceipts(poId)` | `GET /api/purchase-orders/{id}/goods-receipts` |
| `listGoodsReceipts(page)` | `GET /api/goods-receipts` |

### Purchase Returns (`Features/PurchaseReturn`)
| Function reference | Endpoint |
|---|---|
| `createPurchaseReturn(poId, items[], reason)` | `POST /api/purchase-orders/{id}/returns` |
| `getPurchaseReturn(returnId)` | `GET /api/purchase-returns/{id}` |
| `listPurchaseReturns(poId)` | `GET /api/purchase-orders/{id}/returns` |

### Supplier Evaluation (`Features/Supplier`, extended)
| Function reference | Endpoint |
|---|---|
| `evaluateSupplier(supplierId)` | `POST /api/suppliers/{id}/evaluate` |

---

## Request / Response Shapes

### POST `/api/purchase-orders`
```json
{ "supplierId": "...", "items": [{ "itemId": "...", "quantity": 100, "unitCost": 45.50 }] }
```

### POST `/api/purchase-orders/{id}/goods-receipts`
```json
{ "warehouseId": "...", "items": [
    { "purchaseOrderItemId": "...", "acceptedQuantity": 95, "rejectedQuantity": 5 }
] }
```
`rejectedQuantity` defaults to 0 if omitted. `acceptedQuantity` **plus `freeQuantity`** (free-issue
units, default 0) is credited to `StockBalance`; only accepted units are costed and only accepted
units advance the PO line's `ReceivedQuantity`. Optional pricing fields (`unitCost`,
`discountPercent`, `vatPercent`, `nbtPercent` per line; `discountPercent`, `vatPercent`,
`nbtPercent` for the whole GRN) are described in
[refactor_docs/grn_costing_and_taxes.md](./refactor_docs/grn_costing_and_taxes.md).

### POST `/api/purchase-orders/{id}/returns`
```json
{ "reason": "Damaged in transit", "items": [{ "itemId": "...", "quantity": 10 }] }
```

### POST `/api/purchase-orders/{id}/resolve-variance`
```json
{ "variance": "Quantity mismatch: PO 100, GRN 95", "resolution": "Accepted — short shipment",
  "adjustedUnitCost": null, "purchaseOrderItemId": "..." }
```

---

## Business Rules

1. `createPurchaseOrder` is the entry point to procurement — no prior approval step. A PO starts
   in `Issued`.
2. `createGoodsReceipt` must reference an `Issued`/`PartiallyReceived` PurchaseOrder — no receipts
   against a `Closed`/`Cancelled`/nonexistent PO. Each receipt increments
   `PurchaseOrderItem.ReceivedQuantity` by `AcceptedQuantity`, credits `StockBalance` at the
   supplied `warehouseId` (+ a `StockMovement` row, `Type = PurchaseReceipt`), and transitions the
   PO to `PartiallyReceived` or `Received` (every non-cancelled line's `ReceivedQuantity ==
   Quantity`).
3. `closePurchaseOrder` is only valid once a PO is `Received` (or manually closed early with
   outstanding quantity written off); `cancelPurchaseOrder` is only valid before any goods receipt
   exists against the PO.
4. `cancelPurchaseOrderLine` cancels a single `PurchaseOrderItem` (`IsCancelled = true`,
   `CancelledQuantity = Quantity - ReceivedQuantity`) instead of the whole PO. Only valid while
   `ReceivedQuantity < Quantity` and the line isn't already fully received or cancelled. The PO
   auto-completes to `Received` once every non-cancelled line is fully received.
5. `updatePurchaseOrder` may only change unreceived lines (`ReceivedQuantity == 0`) and only while
   the PO is `Issued` — a line that has already received stock, or a PO past `Issued`, cannot be
   edited (use `cancelPurchaseOrderLine`/`createPurchaseReturn` instead).
6. `createPurchaseReturn` requires at least one prior `createGoodsReceipt` against the PO; return
   quantity per item cannot exceed `receivedTotal - alreadyReturned` for that item. Debits
   `StockBalance` at the PO's (most recent) receiving warehouse (+ `StockMovement`,
   `Type = PurchaseReturn`).
7. `matchInvoiceToPO` performs PO-vs-GRN quantity/cost reconciliation only — no `Invoice` entity
   exists yet (step 19 Finance, not built), so `invoiceId` is accepted but not compared against
   anything. Full 3-way match activates once Finance's `Invoice` model exists. Until then it
   compares `PurchaseOrderItem.Quantity`/`UnitCost` against `GoodsReceiptItem.AcceptedQuantity` per
   line and returns a match/variance result — it never blocks or adjusts anything itself.
8. `resolveInvoiceVariance` is the explicit function that records a variance and its resolution
   (accept it, request a corrected invoice, or adjust `PurchaseOrderItem.UnitCost` retroactively) —
   `matchInvoiceToPO` only detects, this function is what acts on a mismatch.
9. `evaluateSupplier` is the only function permitted to write `Supplier.PerformanceScore`
   ([14_pricing_and_suppliers.md](./14_pricing_and_suppliers.md) rule 4) — implemented in
   `SupplierService.EvaluateAsync`, scored from that supplier's PO/GRN history (accepted vs
   rejected quantity ratio, cancelled vs fulfilled line ratio).
10. `SupplierService.DeleteAsync` blocks (soft-)deleting a `Supplier` if any active `PurchaseOrder`
    references it — implemented, closing the previously-open item from
    [14_pricing_and_suppliers.md](./14_pricing_and_suppliers.md).
11. `OrderService.CreateAsync` (`checkStockAvailability`, mainPlan §1) now rejects an order if any
    line's requested quantity exceeds total `StockBalance` for that item summed across every
    location — added alongside this step since it was a documented gap. Does not reserve or
    decrement stock; actual decrement stays deferred to loading/delivery (§04-06).

---

## Permissions to Add to Seed

**Status: Not implemented, matching Supplier/PriceList (step 14).** All procurement controllers use
bare `[Authorize]` (authentication only, no per-action policy) — deliberately kept consistent with
how Supplier and PriceList currently ship rather than introducing enforcement piecemeal. Target
permission codes for when policy enforcement is turned on across steps 14/15 together:
```
procurement.view, procurement.create, procurement.update, procurement.receive, procurement.return,
suppliers.evaluate
```
No role assignments seeded yet — would mirror the pricing/supplier tier (AreaManager (30) and
above) per 14's existing target design.

---

## Open Questions

- Where should cost-vs-sale-price margin reporting (`PurchaseOrderItem.UnitCost` vs
  `PriceListItem.Price`/`Item.SalePrice`) live — a read-only endpoint/report in this step, or a
  later analytics/reporting step? Not a blocker for the core PO→GRN flow.
- Landed cost (freight, duty allocated onto received stock) is out of scope. The GRN now records
  unit cost, discount, NBT and VAT per line (see
  [refactor_docs/grn_costing_and_taxes.md](./refactor_docs/grn_costing_and_taxes.md)), but these do
  not feed stock valuation or `Item.CostPrice`, and the PO itself carries no discount or tax.
- Reorder triggers (`setReorderPoint`/`sendLowStockAlert`) are not wired to procurement — restocking
  stays fully manual until Notifications & Alerts exists.
- `createPurchaseReturn` assumes a single receiving warehouse per PO (uses the most recent
  `GoodsReceipt.FK_WarehouseId`). If a PO is ever received across multiple warehouses, the return
  needs a `warehouseId` on the request instead of inferring one — not needed yet since nothing
  splits a receipt across warehouses today.
- Permission policy enforcement (`procurement.*`) is deferred — see Permissions to Add to Seed.

---

## Implementation Notes

### File structure (as built)
```
Features/
  PurchaseOrder/
    PoController.cs        — /api/purchase-orders (create/get/list/update/close/cancel/
                              cancel-line/match-invoice/resolve-variance)
    PoDTOs.cs               — CreatePoRequest, UpdatePoRequest(+Line), CancelPoRequest(+Line),
                              PoResponse(+Item), InvoiceMatchResult(+Line), ResolveInvoiceVarianceRequest
    PoService.cs            — createPurchaseOrder, updatePurchaseOrder, closePurchaseOrder,
                              cancelPurchaseOrder, cancelPurchaseOrderLine, matchInvoiceToPO,
                              resolveInvoiceVariance
    IPoRepository.cs / PoRepository.cs
  Grn/
    GrnController.cs        — nested under /api/purchase-orders/{id}/goods-receipts,
                              /api/goods-receipts/{id}
    GrnDTOs.cs               — CreateGrnRequest, GrnResponse(+Item)
    GrnService.cs            — createGoodsReceipt (credits StockBalance/StockMovement, updates
                              PurchaseOrderItem.ReceivedQuantity, transitions PO status)
    IGrnRepository.cs / GrnRepository.cs
  PurchaseReturn/
    PurchaseReturnController.cs   — nested under /api/purchase-orders/{id}/returns,
                                     /api/purchase-returns/{id}
    PurchaseReturnDTOs.cs
    PurchaseReturnService.cs      — createPurchaseReturn (debits StockBalance/StockMovement)
    IPurchaseReturnRepository.cs / PurchaseReturnRepository.cs
  Supplier/
    SupplierController.cs   — extended: POST /{id}/evaluate
    SupplierService.cs      — extended: EvaluateAsync (only writer of PerformanceScore),
                              DeleteAsync now blocks if an active PurchaseOrder references the supplier
  Order/
    OrderService.cs          — extended: CreateAsync now calls HasSufficientStockAsync
                              (checkStockAvailability) before resolving price/inserting the order

Models/
  PurchaseOrder.cs, PurchaseOrderItem.cs   (new)
  GoodsReceipt.cs, GoodsReceiptItem.cs     (new)
  PurchaseReturn.cs, PurchaseReturnItem.cs (new)
  Enums.cs                                 (extended — PurchaseOrderStatus enum added)

Data/AppDBContext.cs
  DbSets added for all six entities; FK relationships via Fluent API, DeleteBehavior.Restrict

Migrations/
  20260824110925_AddProcurement   (applied)

Program.cs
  IPoRepository/PoRepository, IGrnRepository/GrnRepository,
  IPurchaseReturnRepository/PurchaseReturnRepository registered; PoService, GrnService,
  PurchaseReturnService registered (Supplier registrations already existed from step 14)
```

### How the models connect
```
Supplier (existing, step 14)
   ▲
   │ FK_SupplierId
PurchaseOrder ──► GoodsReceipt ──► StockBalance/StockMovement (Stock Ledger, §02)
   │                  │               (credited: AcceptedQuantity, MovementType.PurchaseReceipt)
   │                  └─ GoodsReceiptItem ─► PurchaseOrderItem
   │                     (AcceptedQuantity increments stock; also increments
   │                      PurchaseOrderItem.ReceivedQuantity, which drives PurchaseOrder.Status)
   │
   ├─ PurchaseOrderItem (FK_ItemId, Quantity, UnitCost, ReceivedQuantity, CancelledQuantity)
   │
   └─ PurchaseReturn ──► PurchaseReturnItem ──► StockBalance/StockMovement
      (FK_PurchaseOrderId)  (FK_ItemId, Quantity;    (debited: MovementType.PurchaseReturn,
                              via the PO's receiving   at the PO's receiving warehouse)
                              warehouse)

Invoice (Finance, step 19 — not yet built)
   └─ matchInvoiceToPO reconciles PurchaseOrderItem vs GoodsReceiptItem only, today (invoiceId
      accepted but unused); full 3-way match activates once Invoice exists.

evaluateSupplier reads that supplier's PurchaseOrders/GoodsReceipts (accepted-vs-rejected quantity,
cancelled-vs-fulfilled line ratio) and writes Supplier.PerformanceScore — no other function may
write it.
```

### Goods receipt flow (as implemented)
```
GrnService.CreateAsync(purchaseOrderId, request, receivedByUserId):
  po = load PurchaseOrder with Items, must be Issued or PartiallyReceived else throw
  for each (purchaseOrderItemId, acceptedQty, rejectedQty) in request.Items:
    poItem = po.Items.Single(purchaseOrderItemId)
    outstanding = poItem.Quantity - poItem.ReceivedQuantity - poItem.CancelledQuantity
    guard acceptedQty + rejectedQty <= outstanding else throw
    poItem.ReceivedQuantity += acceptedQty
    credit StockBalance(itemId, request.WarehouseId, Warehouse) += acceptedQty
    add StockMovement(Type = PurchaseReceipt, ToLocation = warehouse, ReferenceId = goodsReceipt.Id)
  po.Status = every non-cancelled poItem.ReceivedQuantity == poItem.Quantity
              ? Received : PartiallyReceived
  commit GoodsReceipt + GoodsReceiptItems + updated PurchaseOrderItem rows + StockBalance/Movement
  in one transaction (IUnitOfWork.Begin/Commit)
```

### Purchase return flow (as implemented)
```
PurchaseReturnService.CreateAsync(purchaseOrderId, request, createdByUserId):
  po = load PurchaseOrder, must have >= 1 GoodsReceipt else throw
  warehouseId = most recent GoodsReceipt.FK_WarehouseId for this Po
  for each (itemId, qty) in request.Items:
    received = sum(GoodsReceiptItem.AcceptedQuantity for itemId across po's receipts)
    alreadyReturned = sum(PurchaseReturnItem.Quantity for itemId across po's prior returns)
    guard qty <= received - alreadyReturned else throw
    debit StockBalance(itemId, warehouseId) -= qty
    add StockMovement(Type = PurchaseReturn, FromLocation = warehouseId, ReferenceId = purchaseReturn.Id)
  commit PurchaseReturn + PurchaseReturnItems + StockBalance/Movement in one transaction
```