Credit balance on my account and wanting to Close it

I just recently paid up my DIonwired account via my FNB Loan and now have a credit balance. I requested to close my account and they need to refund what I paid in excess. I cannot provide a receipt of the last payment as it was paid via my loan. The payments reflect on the account statement, however RCS is asking me for proof and WHY I overpaid???? Just pay me what I am owed.