Over payment

I finished the repayments on a vehicle and I was then informed that I overpaid an amount to MFC. After weeks of struggling to get the over payment back, and several visits to their offices and promises of returned phone calls, I have not succeeded. Yesterday I was informed that the repayment can only be done once I have provided proof that I indeed made the over payment - this after they informed me of the over payment. I refuse to provide MFC with my bank statements to show the amount(s) paid to them. I am not applying for finance with them, so they have no right to my bank statements. The over payment must be reflected on their billing system. Despite promises that they will call me back, I have not received any feedback since yesterday.