Payment not allocated

Very bad service from Foschini. I have been trying to sort out an account that I had paid in full in 2016 but Foschini still claims is outstanding. I have the letter of confirmation of payment from their collections department, but everyone that I have spoken to (I have phoned several different departments) says it is someone else's problem to sort out. Do I have to pay Foschini again in order to settle this, or contact an attorney to sort this out? Either way, it is going to cost me, the consumer, extra money to sort out their mistakes!