fix: don't reset rate if greater than zero in standalone debit note (backport #37935) (#37940)

* fix: don't reset rate if greater than zero in standalone debit note

(cherry picked from commit 5cce522ecd)

* fix(test): `test_gl_entries_for_standalone_debit_note`

(cherry picked from commit f9fc6c9c9d)

---------

Co-authored-by: s-aga-r <sagarsharma.s312@gmail.com>

erpnext/accounts/doctype/purchase_invoice/test_purchase_invoice.py

@@ -1718,9 +1718,14 @@ class TestPurchaseInvoice(FrappeTestCase, StockTestMixin):
 		self.assertTrue(return_pi.docstatus == 1)
 
 	def test_gl_entries_for_standalone_debit_note(self):
-		make_purchase_invoice(qty=5, rate=500, update_stock=True)
+		from erpnext.stock.doctype.item.test_item import make_item
 
-		returned_inv = make_purchase_invoice(qty=-5, rate=5, update_stock=True, is_return=True)
+		item_code = make_item(properties={"is_stock_item": 1})
+		make_purchase_invoice(item_code=item_code, qty=5, rate=500, update_stock=True)
+
+		returned_inv = make_purchase_invoice(
+			item_code=item_code, qty=-5, rate=5, update_stock=True, is_return=True
+		)
 
 		# override the rate with valuation rate
 		sle = frappe.get_all(
@@ -1730,7 +1735,7 @@ class TestPurchaseInvoice(FrappeTestCase, StockTestMixin):
 		)[0]
 
 		rate = flt(sle.stock_value_difference) / flt(sle.actual_qty)
-		self.assertAlmostEqual(returned_inv.items[0].rate, rate)
+		self.assertAlmostEqual(rate, 500)
 
 	def test_payment_allocation_for_payment_terms(self):
 		from erpnext.buying.doctype.purchase_order.test_purchase_order import (

erpnext/controllers/buying_controller.py

@@ -78,7 +78,7 @@ class BuyingController(SubcontractingController):
 	def set_rate_for_standalone_debit_note(self):
 		if self.get("is_return") and self.get("update_stock") and not self.return_against:
 			for row in self.items:
-
+				if row.rate <= 0:
 					# override the rate with valuation rate
 					row.rate = get_incoming_rate(
 						{