Refund Amount subtract from credit amount

If a customer purchase item on credit and refunds it or replace it its amount should be subtracts from original amount but its shows the bill in full amount.

Like I bought 2 things

Book amount 200

Shirt 500

Total is 700

And I return book amount 200 but after it show the bill I full amount of 700, it should automatically restore or adjust the bill payable. It restocks the items but bill is not adjusted. Plz guide on this.

Thank you