FNB dishonest with personal loan charges/credits

I recently settled a personal loan that I had with FNB on 03 April. I have received my statement indicating that I do not owe them a cent anymore, but that I am actually in credit with an amount of R129. 64CR. When enquiring about transferring the credit balance to my account, I was told that the credit is not due to me, but that it is for interest. I do not understand what interest, as the account is paid in full.... actually overpaid. Now if FNB does this to every loan holder, how many millions are they not *****ING from public. I want my refund and a response as a matter of urgency . FNB you are a bunch of ****s.