Feeling cheated

I received a 60% discount on my Standard Bank credit card and I paid all the money as per settlement in January, I sent them the proof of payment as requested so that they can send me a paid up letter instead they sent me a letter of demand on the discounted amount with a crazy interest added on. I called multiple times but cannot find joy. They keep on promising to send me a paid up letter and promising to call me back but with never. I owed CC R52000 with 60% discount I paid R20000+ now the new letter of demand says I owe R32000 plus interest that amounts to R45000...how does one deal with this