Paid account using the wrong reference number

I overpaid on my Foschini account via EFT. Unfortunately when I setup the beneficiary I used the wrong reference number. I have notified Foschini's via e-mail and telephone. I have sent them the proof of the payments with the wrong account number. Now I have been handed over to the attorneys for R280, when in fact I overpaid with R320. Please I need to rectify this. I have never been in arrears on any of my accounts.