Refund and paid up letter

Upon receipt of my settlement letter for the outstanding balance on my FNB Loan account, I've settled this account on the 28th March and sent them the proof of payment.

When I visited the FNB branch the next day, I spoken to them telephonically requesting a paid up letter from them confirming that the account has been settled and close. They've advised me that they need to refund me as the amount I've paid was more than the settlement amount.

Up to know I'm still waiting for this refund and the paid up letter. Why is taking so long to finalise this?