Cheated

I recently took a personal loan with Nedbank which was used to settle an existing loan with Nedbank and RCS.

It happened that a debit order was paid for both loans from my account (R1541 for Nedbank and R2532 fro RCS) before I could get the new loan. When both loans were settled I had a credit balance of R1973 for my RCS loan becuase of the debit order. However Nedbank is refusing to admit that I have a credit balance with them as well. Instead they are charging double interests.