Poor service delivery

On 25/11/2021 I was requested to pay the amount of R5608.17 but if I settle the account that same day, I would received a discount of 20%, which I did on 25/11/2021, and the amount of R 4644.00 was paid on 25/11/2021. This was to settle the amount in full, and I received the paid in full letter from standard bank on 10/12/2021 from Diana Nkosi of Bham & Dahya attorneys .

Unfortunately Debt Busters made another payment to the fully paid account on 01/12/2021 to the amount of R2012.24, to which I immediately requested the refund of this amount and received the following reply on 24/01/2022:

Our record shows the balance was R 5806.17 by the time we received the payment of R 4644.00.This means there was a short fall of R 1 162.17 to settle the account . We then received another payment of R 2012.24 on the 02/12/2022 which covered the shortfall of R 1162.17. The account was then in credit with 847.41. The account then billed on the 03/01/2022 ,interest and fees of R40 were charged ,that left your credit card with a credit balance of R 807.48. Hence the transferred of R 807.48 was refunded to you.

How can I firstly be billed with interest and fees if the fully paid letter was received and the account closed? I requested the balance of the payment they received in error to be refunded to me, but to date am still trying to get any response from them whatsoever.