standardbank says I owe them

I was handed over by standardbank for credit card dept. After pay every month as agreed, a person called me with an offer of 20 percent discount if I settled the account in full. U did pay the full discounted amount BEFORE the agreed date. Since then I got calls again saying I owe them. After investigations by a person from standard brooklyn branch we found that the teller inserted the wrong ref. No. On the computer resulting in the funds not being a located to the correct account. This has been corrected but I am still receiving calls about an outstanding amount. Exactly the amount that I was offered as discount. What do I still have to do to get this cleared up? Standardbank was clearly at fault for the mistake.