Merge pull request #53925 from ljain112/fix-item-tax-rounding

erpnext/controllers/taxes_and_totals.py

@@ -285,6 +285,13 @@ class calculate_taxes_and_totals:
 		self.doc._item_wise_tax_details = item_wise_tax_details
 		self.doc.item_wise_tax_details = []
 
+		for tax in self.doc.get("taxes"):
+			if not tax.get("dont_recompute_tax"):
+				tax._running_txn_tax_total = 0.0
+				tax._running_base_tax_total = 0.0
+				tax._running_txn_taxable_total = 0.0
+				tax._running_base_taxable_total = 0.0
+
 	def determine_exclusive_rate(self):
 		if not any(cint(tax.included_in_print_rate) for tax in self.doc.get("taxes")):
 			return
@@ -521,7 +528,6 @@ class calculate_taxes_and_totals:
 			actual_breakup = tax._total_tax_breakup
 			diff = flt(expected_amount - actual_breakup, 5)
 
-			# TODO: fix rounding difference issues
 			if abs(diff) <= 0.5:
 				detail_row = self.doc._item_wise_tax_details[last_idx]
 				detail_row["amount"] = flt(detail_row["amount"] + diff, 5)
@@ -597,14 +603,25 @@ class calculate_taxes_and_totals:
 	def set_item_wise_tax(self, item, tax, tax_rate, current_tax_amount, current_net_amount):
 		# store tax breakup for each item
 		multiplier = -1 if tax.get("add_deduct_tax") == "Deduct" else 1
-		item_wise_tax_amount = flt(
-			current_tax_amount * self.doc.conversion_rate * multiplier, tax.precision("tax_amount")
+
+		# Error diffusion: derive each item's base amount as a delta of the running cumulative total
+		# so the sum always equals base_tax_amount_after_discount_amount.
+		tax._running_txn_tax_total += current_tax_amount * multiplier
+		new_base_tax_total = flt(
+			flt(tax._running_txn_tax_total, tax.precision("tax_amount")) * self.doc.conversion_rate,
+			tax.precision("base_tax_amount"),
 		)
+		item_wise_tax_amount = new_base_tax_total - tax._running_base_tax_total
+		tax._running_base_tax_total = new_base_tax_total
 
 		if tax.charge_type != "On Item Quantity":
-			item_wise_taxable_amount = flt(
-				current_net_amount * self.doc.conversion_rate * multiplier, tax.precision("tax_amount")
+			tax._running_txn_taxable_total += current_net_amount * multiplier
+			new_base_taxable_total = flt(
+				flt(tax._running_txn_taxable_total, tax.precision("net_amount")) * self.doc.conversion_rate,
+				tax.precision("base_net_amount"),
 			)
+			item_wise_taxable_amount = new_base_taxable_total - tax._running_base_taxable_total
+			tax._running_base_taxable_total = new_base_taxable_total
 		else:
 			item_wise_taxable_amount = 0.0
 

erpnext/controllers/tests/test_item_wise_tax_details.py

@@ -1,6 +1,6 @@
 import frappe
 
-from erpnext.tests.utils import ERPNextTestSuite
+from erpnext.tests.utils import ERPNextTestSuite, change_settings
 
 
 class TestTaxesAndTotals(ERPNextTestSuite):
@@ -121,3 +121,124 @@ class TestTaxesAndTotals(ERPNextTestSuite):
 		]
 
 		self.assertEqual(actual_values, expected_values)
+
+	@change_settings("Selling Settings", {"allow_multiple_items": 1})
+	def test_item_wise_tax_detail_high_conversion_rate(self):
+		"""
+		With a high conversion rate (e.g. USD -> KRW ~1300), independently rounding
+		each item's base tax amount causes per-item errors that accumulate and exceed
+		the 0.5-unit safety threshold, raising a validation error.
+
+		Error diffusion fixes this: the cumulative base total after the last item
+		equals base_tax_amount_after_discount_amount exactly, so the sum of all
+		per-item amounts is always exact regardless of item count or rate magnitude.
+
+		Analytically with conversion_rate=1300, rate=7.77 x3 items, VAT 16%:
+		per-item txn tax = 1.2432
+		OLD independent: flt(1.2432 * 1300, 2) = 1616.16 -> sum 4848.48
+		expected base:   flt(flt(3.7296, 2) * 1300, 0) = flt(3.73 * 1300, 0) = 4849
+		diff = 0.52 -> exceeds 0.5 threshold -> would throw with old code
+		"""
+		doc = frappe.get_doc(
+			{
+				"doctype": "Sales Invoice",
+				"customer": "_Test Customer",
+				"company": "_Test Company",
+				"currency": "USD",
+				"debit_to": "_Test Receivable USD - _TC",
+				"conversion_rate": 1300,
+				"items": [
+					{
+						"item_code": "_Test Item",
+						"qty": 1,
+						"rate": 7.77,
+						"income_account": "Sales - _TC",
+						"expense_account": "Cost of Goods Sold - _TC",
+						"cost_center": "_Test Cost Center - _TC",
+					},
+					{
+						"item_code": "_Test Item",
+						"qty": 1,
+						"rate": 7.77,
+						"income_account": "Sales - _TC",
+						"expense_account": "Cost of Goods Sold - _TC",
+						"cost_center": "_Test Cost Center - _TC",
+					},
+					{
+						"item_code": "_Test Item",
+						"qty": 1,
+						"rate": 7.77,
+						"income_account": "Sales - _TC",
+						"expense_account": "Cost of Goods Sold - _TC",
+						"cost_center": "_Test Cost Center - _TC",
+					},
+				],
+				"taxes": [
+					{
+						"charge_type": "On Net Total",
+						"account_head": "_Test Account VAT - _TC",
+						"cost_center": "_Test Cost Center - _TC",
+						"description": "VAT",
+						"rate": 16,
+					},
+					{
+						"charge_type": "On Previous Row Amount",
+						"account_head": "_Test Account Service Tax - _TC",
+						"cost_center": "_Test Cost Center - _TC",
+						"description": "Service Tax",
+						"rate": 10,
+						"row_id": 1,
+					},
+				],
+			}
+		)
+		doc.save()
+
+		details_by_tax = {}
+		for detail in doc.item_wise_tax_details:
+			bucket = details_by_tax.setdefault(detail.tax_row, 0.0)
+			details_by_tax[detail.tax_row] = bucket + detail.amount
+
+		for tax in doc.taxes:
+			self.assertEqual(details_by_tax[tax.name], tax.base_tax_amount_after_discount_amount)
+
+	def test_item_wise_tax_detail_with_multi_currency_with_single_item(self):
+		"""
+		When the tax amount (in transaction currency) has more decimals than
+		the field precision, rounding must happen *before* multiplying by
+		conversion_rate — the same order used by _set_in_company_currency.
+		"""
+		doc = frappe.get_doc(
+			{
+				"doctype": "Sales Invoice",
+				"customer": "_Test Customer",
+				"company": "_Test Company",
+				"currency": "USD",
+				"debit_to": "_Test Receivable USD - _TC",
+				"conversion_rate": 129.99,
+				"items": [
+					{
+						"item_code": "_Test Item",
+						"qty": 1,
+						"rate": 47.41,
+						"income_account": "Sales - _TC",
+						"expense_account": "Cost of Goods Sold - _TC",
+						"cost_center": "_Test Cost Center - _TC",
+					}
+				],
+				"taxes": [
+					{
+						"charge_type": "On Net Total",
+						"account_head": "_Test Account VAT - _TC",
+						"cost_center": "_Test Cost Center - _TC",
+						"description": "VAT",
+						"rate": 16,
+					},
+				],
+			}
+		)
+		doc.save()
+
+		tax = doc.taxes[0]
+		detail = doc.item_wise_tax_details[0]
+		self.assertEqual(detail.amount, tax.base_tax_amount_after_discount_amount)