Charging customers interest after they have settled the account

I got a settlement letter on the 15th of December 2021 from direct axis and paid it on the 16th December 2021 to my surprise I now received a statement showing that I owe R200. 49 but the account was settled. I even called early this month and was promised that it will be closed. I would like to get my refund back and explanation in writing of why the account is still owing meanwhile I received a settlement letter from them. I feel that you are taking your customers for a ride and charging ******* interest. Im going to take this matter to the ombudsman