fix: rate changing while making PR (negative discount) (#40539)

diff --git a/erpnext/public/js/controllers/taxes_and_totals.js b/erpnext/public/js/controllers/taxes_and_totals.js
index 3099184..4a525b2 100644
--- a/erpnext/public/js/controllers/taxes_and_totals.js
+++ b/erpnext/public/js/controllers/taxes_and_totals.js
@@ -26,7 +26,7 @@
 			item.discount_amount = flt(item.rate_with_margin) * flt(item.discount_percentage) / 100;
 		}
 
-		if (item.discount_amount) {
+		if (item.discount_amount > 0) {
 			item_rate = flt((item.rate_with_margin) - (item.discount_amount), precision('rate', item));
 			item.discount_percentage = 100 * flt(item.discount_amount) / flt(item.rate_with_margin);
 		}