Biggest ********/******

MBD sent me an sms with a balance of my foschini acc, I went and paid it in full, a week later they called me and told me I owe them R89 for interest, where does the interest come from if my account is on 00.00? I called them and some to a very rude consultant who told me that I will have to pay that amount or my interest will keep growing. These people are robbing people really.