fix(medusa): Discount allocation precision issues (#3244)

packages/medusa/src/services/new-totals.ts

@@ -172,8 +172,7 @@ export default class NewTotalsService extends TransactionBaseService {
       subtotal = 0 // in that case we need to know the tax rate to compute it later
     }
 
-    const discount_total =
-      (lineItemAllocation.discount?.unit_amount || 0) * item.quantity
+    const discount_total = lineItemAllocation.discount?.amount ?? 0
 
     const totals: LineItemTotals = {
       unit_price: item.unit_price,
@@ -275,8 +274,7 @@ export default class NewTotalsService extends TransactionBaseService {
       subtotal = 0 // in that case we need to know the tax rate to compute it later
     }
 
-    const discount_total =
-      (lineItemAllocation.discount?.unit_amount || 0) * item.quantity
+    const discount_total = lineItemAllocation.discount?.amount ?? 0
 
     const totals: LineItemTotals = {
       unit_price: item.unit_price,
@@ -428,8 +426,7 @@ export default class NewTotalsService extends TransactionBaseService {
         )
 
     const discountAmount =
-      (calculationContext.allocation_map[lineItem.id]?.discount?.unit_amount ||
-        0) * lineItem.quantity
+      calculationContext.allocation_map[lineItem.id]?.discount?.amount ?? 0
 
     const lineSubtotal =
       (lineItem.unit_price - taxAmountIncludedInPrice) * lineItem.quantity -

packages/medusa/src/services/totals.ts

@@ -462,12 +462,18 @@ class TotalsService extends TransactionBaseService {
         if (allocationMap[ld.item.id]) {
           allocationMap[ld.item.id].discount = {
             amount: adjustmentAmount,
+            /**
+             * Used for the refund computation
+             */
             unit_amount: Math.round(adjustmentAmount / ld.item.quantity),
           }
         } else {
           allocationMap[ld.item.id] = {
             discount: {
               amount: adjustmentAmount,
+              /**
+               * Used for the refund computation
+               */
               unit_amount: Math.round(adjustmentAmount / ld.item.quantity),
             },
           }
@@ -794,8 +800,7 @@ class TotalsService extends TransactionBaseService {
       subtotal = 0 // in that case we need to know the tax rate to compute it later
     }
 
-    const discount_total =
-      (lineItemAllocation.discount?.unit_amount || 0) * lineItem.quantity
+    const discount_total = lineItemAllocation.discount?.amount ?? 0
 
     const lineItemTotals: LineItemTotals = {
       unit_price: lineItem.unit_price,

packages/medusa/src/strategies/tax-calculation.ts

@@ -73,9 +73,7 @@ class TaxCalculationStrategy implements ITaxCalculationStrategy {
         taxableAmount = item.unit_price * item.quantity
       }
 
-      taxableAmount -=
-        ((allocations.discount && allocations.discount.unit_amount) || 0) *
-        item.quantity
+      taxableAmount -= allocations.discount?.amount ?? 0
 
       for (const filteredTaxLine of filteredTaxLines) {
         taxTotal += Math.round(