How can we milk you?

The balance on my credit card account on 26/08/20121 was R291-00 which was payable on 20/09/2021. FNB stated on my statement that R192-00 will be debited to my account on due date. I therefore made a payment of R100-00 to my account which was shown on 18/09/2021. Somebody paid R50-00 into my account without telling me. If the debit was processed as indicated my account would have been overpaid by R51-00. FNB decided to only debit my bank account with R42-00 which, with the other payments, totaled R192-00 which was the minimum payment required. They therefore decided to ignore that I would have paid the full balance if they did not change their charges. As the made my account to be in arrears they duly charged me on interest on the the account that they caused to be in arrears,