fix: incorrect outstanding on non-pos invoice with write_off_account

(cherry picked from commit d5e2906)

erpnext/accounts/doctype/sales_invoice/sales_invoice.py

@@ -340,6 +340,7 @@ def validate(self):
 		):
 			validate_loyalty_points(self, self.loyalty_points)
 
+		self.allow_write_off_only_on_pos()
 		self.reset_default_field_value("set_warehouse", "items", "warehouse")
 
 	def validate_accounts(self):
@@ -1021,6 +1022,10 @@ def validate_delivery_note(self):
 					raise_exception=1,
 				)
 
+	def allow_write_off_only_on_pos(self):
+		if not self.is_pos and self.write_off_account:
+			self.write_off_account = None
+
 	def validate_write_off_account(self):
 		if flt(self.write_off_amount) and not self.write_off_account:
 			self.write_off_account = frappe.get_cached_value("Company", self.company, "write_off_account")