I'm Harrased By Attorney With Account that I Paid

I had an account with Woolworths and unfortunately it was not handled well until it was handed over to MBD attorney's. I then made an arrangement with them of R500 monthly repayments for two years which the last payment of R145 was paid end of February of 2010. I was shocked and surprised to receive a call from MBD attorneys after a year I had finishned paying the account. They guy who approached me on the phone said they made a mistake by not adding interest on the installments I paid, therefore I have to pay R750 which is interest. I then asked the gentlemen why was the interested not added on the monthly repayment yet they had been given liberty to debit my bank account monthly, unfortunately he could not give me an answer. So after failing in stealing my money they then sold files to A de vos Attorney's who is harassing me on daily basis about this money threatening me about handing over the account and garnish my salary for the account that I paid in full 6 years ago. These people are making money out of people who are working hard for their families. Mrs Bailey told me that I'm now owing R1500 on this account because this interest has also accumulated another interest