MFC- PAY BACK THE MONEY

I was behind with my car payment, called MFC to make arrangements with them which we agreed upon. Then later on they called requesting me to make a cash payment deposit of R500, and when they debit my account they will debit it with less R500 which i would have paid cash into their account through Nedbank. On the 30th of April I went and deposited the cash as per agreement. When they had to debit my account they debited the full amount and not less R500 as per our agreement. I called them about this and requested that they pay back the R500 since they took their full amount, this is where the stories started. They confirmed that the amount of R500 reflects on my account, but they requested me to send them my bank statement so that they can transfer the money. I sent them my bank statement but no money was transferred. I called again to make a follow up, I was told i needed to send them proof of payment. I told them i had misplaced the proof of payment, I was then told that i must go to the Nedbank i had made the payment to and request for proof of payment. I went to Nedbank Lifestyle Centre in Centurion, and they told me on the day I made this transfer the teller's computer who helped me had crushed and therefore could not assist me. I called MFC to explain my situation, I was then told that they will send it to they repayment departemnt and the money will take 10 working days to be transferred because i am not using Nedbank. Weeks passed without money into my account, I once again called and this time I was told that this R500 has been put into my last installment of my car repayment. I told them I don't want it like that I want my money back. I was once again promised the money will be transferred, but it never was. When I call again yesterday the 11th of April, they are telling me they can't reimburse me if I don't have proof of payment. I do't understand why when I have explained to them a number of times that I do not have proof of this payment. And the money reflects in my account. All I want from MFC is for them to pay me back my R500. Had it been me that was owing them I would not be sleeping at night because they would be calling me day in and day out, with interest on the money. So, please assist me in getting my R500 back from MFC. Clearly for a huge company like them, the can afford to pay R500 to a poor person like me.