African bank is trying to **** me

I had a loan with african bank,which I paid upbin September, mind you before I paid I requested the paid up letter, which I paid the exact amount they requested me to on the letter, on the 25th of September they tried to debit R75 but there was no money so it was unsuccessful, today I got a notification from my bank saying that I must give african bank approval to debit R400 from my account, why are doing this