@@ -17,6 +17,7 @@ import {
 } from '@/lib/cost-layers'
 import { sliceTransferSnapshotForReceipt } from '@/lib/domain/wms/asn-reconciliation'
 import { toInventoryConstraintMessage } from '@/lib/domain/inventory/prisma-errors'
+import { canDispatchTransferQty } from '@/lib/domain/inventory/transfer-availability'
 import { addMoney, multiplyMoney, roundQuantity, toDecimal } from '@/lib/domain/math/decimal'
 import { serializeCostLayerSnapshot } from '@/lib/cost-layer-snapshots'
 import {
@@ -388,9 +389,13 @@ export async function dispatchTransfer(id: string): Promise<TransferResult> {
           where: { productId_warehouseId: { productId: line.productId, warehouseId: transfer.fromWarehouseId } },
           select: { quantity: true, reservedQty: true },
         })
-        const available = level ? Number(level.quantity) - Number(level.reservedQty) : 0
-        if (available < qty) {
-          throw new Error(`Insufficient stock for ${line.sku}: ${available} available, ${qty} requested`)
+        // audit-M-stock #1: net the source warehouse's reserved (allocated)
+        // quantity so a transfer can't drain stock an order is holding there.
+        if (!canDispatchTransferQty(level?.quantity, level?.reservedQty, qty)) {
+          // Report the raw (unclamped) delta so an over-reservation (negative)
+          // stays a visible diagnostic, not silently shown as 0.
+          const rawAvailable = Number(level?.quantity ?? 0) - Number(level?.reservedQty ?? 0)
+          throw new Error(`Insufficient stock for ${line.sku}: ${rawAvailable} available, ${qty} requested`)
         }
       }
 
 
@@ -151,3 +151,29 @@ refund payments.
 
 Stock transfer status tracks inter-warehouse movement. `IN_TRANSIT` means stock
 has left the source warehouse and is not yet available at the destination.
+
+**Stock / concurrency invariants (audit-M-stock).** A few cross-cutting guards
+worth knowing:
+
+- **Transfers don't strand allocations.** Dispatch availability is the source
+  warehouse's on-hand minus its `reservedQty` (`availableForTransfer`), and
+  `StockLevel.reservedQty` is per-(product, warehouse) and kept in sync with
+  order allocations — so a transfer can never drain stock an order is holding in
+  that warehouse.
+- **Manual receipt + WMS booked-in don't double-count.** `reconcileBookedInQuantities`
+  nets the locally-received quantity (read under a `FOR UPDATE` lock on the PO or
+  the stock transfer) so a manual receipt already covering a line is not re-added
+  when the Mintsoft booked-in webhook approves the same ASN.
+- **Opening stock can't duplicate.** `applyOpeningStock` takes a `FOR UPDATE`
+  lock on the stock level before checking for an existing opening cost layer, so
+  concurrent calls serialise and the second is rejected.
+- **Non-negative stock in the DB.** `quantity >= 0` and `reservedQty >= 0` are
+  fully VALIDATEd CHECK constraints (every existing row checked). `reservedQty <=
+  quantity` is a `NOT VALID` constraint — enforced on new writes but not yet
+  validated against historical rows (that cleanup is deferred), so the transfer
+  guard above (not just the constraint) is what protects allocations at dispatch.
+- **FIFO ordering at the destination** of a received transfer follows the
+  dispatch-time cost-layer snapshot order. If transfers are received out of
+  dispatch order the recreated layers can be marginally out of strict FIFO order
+  at the destination — this is cosmetic for reporting and accepted (the cost
+  basis per layer is preserved).
@@ -0,0 +1,37 @@
+// decimal-boundary-ok: server-action-boundary (numeric stock availability check)
+import { decimalToNumber, type DecimalLike } from '@/lib/decimal'
+
+// ---------------------------------------------------------------------------
+// Transfer-dispatch availability (audit-M-stock #1)
+//
+// A transfer must not drain stock that an order has already reserved in the
+// SOURCE warehouse, or the order is stranded. StockLevel.reservedQty is
+// per-(product, warehouse) and kept in sync with order allocations, so the
+// available-to-transfer quantity is the warehouse's on-hand minus its reserved
+// — netting out exactly the allocations pointing at that warehouse. Centralised
+// here so the rule is explicit and regression-tested rather than an inline
+// subtraction.
+// ---------------------------------------------------------------------------
+
+/**
+ * On-hand minus reserved for a single (product, warehouse) stock level, clamped
+ * to >= 0. Correct for dispatch gating; do NOT reuse for data-integrity checks —
+ * the clamp hides an over-reservation (reservedQty > quantity), which raw
+ * subtraction would surface as a negative.
+ */
+export function availableForTransfer(
+  quantity: DecimalLike | null | undefined,
+  reservedQty: DecimalLike | null | undefined,
+): number {
+  const available = decimalToNumber(quantity ?? 0) - decimalToNumber(reservedQty ?? 0)
+  return available > 0 ? available : 0
+}
+
+/** Whether `requestedQty` can be dispatched without eating into reserved (allocated) stock. */
+export function canDispatchTransferQty(
+  quantity: DecimalLike | null | undefined,
+  reservedQty: DecimalLike | null | undefined,
+  requestedQty: number,
+): boolean {
+  return requestedQty <= availableForTransfer(quantity, reservedQty)
+}
@@ -0,0 +1,32 @@
+import assert from 'node:assert/strict'
+import test from 'node:test'
+
+import { Prisma } from '@/app/generated/prisma/client'
+import { availableForTransfer, canDispatchTransferQty } from '@/lib/domain/inventory/transfer-availability'
+
+test('availableForTransfer nets reserved (allocated) qty out of on-hand', () => {
+  assert.equal(availableForTransfer(new Prisma.Decimal('100'), new Prisma.Decimal('30')), 70)
+})
+
+test('availableForTransfer never goes negative', () => {
+  assert.equal(availableForTransfer(new Prisma.Decimal('10'), new Prisma.Decimal('15')), 0)
+})
+
+test('availableForTransfer treats missing level as zero', () => {
+  assert.equal(availableForTransfer(null, null), 0)
+  assert.equal(availableForTransfer(undefined, undefined), 0)
+})
+
+test('canDispatchTransferQty: cannot dispatch into reserved stock', () => {
+  // 100 on hand, 30 reserved for an order → only 70 transferable.
+  assert.equal(canDispatchTransferQty(new Prisma.Decimal('100'), new Prisma.Decimal('30'), 70), true)
+  assert.equal(canDispatchTransferQty(new Prisma.Decimal('100'), new Prisma.Decimal('30'), 71), false)
+})
+
+test('canDispatchTransferQty: full unreserved stock is transferable', () => {
+  assert.equal(canDispatchTransferQty(new Prisma.Decimal('50'), new Prisma.Decimal('0'), 50), true)
+})
+
+test('canDispatchTransferQty: rejects when over-reserved (reservedQty > quantity)', () => {
+  assert.equal(canDispatchTransferQty(new Prisma.Decimal('10'), new Prisma.Decimal('15'), 1), false)
+})