Billing nightmare

On the 31st July 2021 I paid my account via the MyMtn App and for some reason the amount was not allocated. I then raised the matter with MTN and provided proof of payment as well as the confirmation email that I received from them and requested that they allocate the amount. However, for some weird reason someone at MTN decided to process a debit order which returned unsuccessful as I had moved funds from my bank account. This unsuccessful debit order resulted in a fee for unpaid items being added to the August 2021 invoice. I again raised this matter with MTN and indicated that I'm not going to pay that extra amount because I did everything correct from my side and instead they gave me a middle finger saying "Client is liable". I am not going to pay that additional amount, if they want a fight then so be it.