[Fix] Allow to change exchange rate in the payment entry if allow stale is enabled (#12128)

diff --git a/erpnext/accounts/doctype/payment_entry/payment_entry.js b/erpnext/accounts/doctype/payment_entry/payment_entry.js
index d8995a9..c5018e2 100644
--- a/erpnext/accounts/doctype/payment_entry/payment_entry.js
+++ b/erpnext/accounts/doctype/payment_entry/payment_entry.js
@@ -408,7 +408,7 @@
 		}
 
 		// Make read only if Accounts Settings doesn't allow stale rates
-		frm.set_df_property("source_exchange_rate", "read_only", erpnext.stale_rate_allowed());
+		frm.set_df_property("source_exchange_rate", "read_only", erpnext.stale_rate_allowed() ? 0 : 1);
 	},
 
 	target_exchange_rate: function(frm) {
@@ -429,7 +429,7 @@
 		frm.set_paid_amount_based_on_received_amount = false;
 
 		// Make read only if Accounts Settings doesn't allow stale rates
-		frm.set_df_property("target_exchange_rate", "read_only", erpnext.stale_rate_allowed());
+		frm.set_df_property("target_exchange_rate", "read_only", erpnext.stale_rate_allowed() ? 0 : 1);
 	},
 
 	paid_amount: function(frm) {