Dishonesty / debiting even after the account is fully paid

On the 13/08 I paid the balance on my Direct Axis account and after that on the FNB app it showed that I still owe R2950 which I immediately paid. To my surprise my I was still debited on the 27th of Aug, when I enquired about that I was told that they are still going to debit my account because I paid the balance without letting them know. When I requested a paid up letter, I was told that I need to send proof of payment even though they are telling me that they've received the full amount. I sent all the documents they required from me but still I haven't received my paid up letter and it's still says I owe them even after my account was fully paid and even more. When I call them they say I will receive it after 24 hours but it has been over a week now. This is really frustrating and it is damaging my credit score