fix: rate change on changing of the qty (backport #40241) (#40243)

fix: rate change on changing of the qty (#40241)

(cherry picked from commit e7d7077)

Co-authored-by: rohitwaghchaure <rohitw1991@gmail.com>

erpnext/public/js/controllers/taxes_and_totals.js

@@ -22,7 +22,7 @@ erpnext.taxes_and_totals = class TaxesAndTotals extends erpnext.payments {
 
 		item_rate = flt(item.rate_with_margin , precision("rate", item));
 
-		if (item.discount_percentage) {
+		if (item.discount_percentage && !item.discount_amount) {
 			item.discount_amount = flt(item.rate_with_margin) * flt(item.discount_percentage) / 100;
 		}