Paid up account but MFC refusing to change ownersh

I bought a Polo GTi 2008 model through NEDBANK/MFC finance and have been paying my account till the last contractual pay date which was on the 25/03/2015. Throughout this period, NEDBANK/MFC have been debiting a monthly installment of R5600 from my account up until the end of March 2015 and since then they have stopped to do so as the balance has been settled. To my surprise, when I inquired about the documentation and change of ownership, MFC tells me that I owe them an amount of R5670.00. When I try to find out about this problem, I'm being sent from one person to another. This afternoon, (10/07/2015 14:59 pm) they send me an SMS demanding that I pay the aforementioned amount before the 15/07/2015, or else they threaten to list me on ITC as a bad customer. I believe that MFC/NEDBANK are trying to rip me off with this ghost amount. Please help!