AR Bank Charges

We are trying to capture Bank charges for Factored receipts in Account receivable module(R12.1.3) but system is not allowing me to enter bank charge amount. The field is grayed out.

However for Standard receipt We were able to capture bank Charges but system is not considering that amount while calculating final due. Following scenario below

There is a Transaction of 1000 $. We have received 50% i.e $500  and bank is deducting $10  as Bank charges. As Per our expectation If I create a receipt with Net receipt Amount 490 and Bank charges 10. There should be due of 500 $ as net balance for receipt but system is showing 510 $ as due amount.