How to make payments to unapplied credits and unapplied receipts

Customer account appears with an invoice of $50.00 and an unapplied receipt of $10.40. So, the total owed by the Customer is $39.60 and that is what should be taken by PAD(Pre Authorized Debit). When the Installment Due Date Report is run it shows the amount that will be taken by PAD to be $50.00, not the Customer’s balance due, which is incorrect.

Please provide a solution to this