Refund of wrong payment

I accidentally paid the wrong amount into my Edgar’s account, I paid R7000-00 instead of R700-00. I did report this immediately and they informed me that the amount of R6300-00 will be refunded but I need to send them some document, which I did. They confirmed they received all the relevant document, just to phone me back few days later and say that they won’t be able to refund me because there Is still an outstanding amount on my account. This is unacceptable as I would never be able to pay R7000-00 into my Edgar’s account in one month, I have a contract that states that there is a minimum amount that I must pay back every month for a period of 12 months. Not once did they even say that there is a possibility that they won’t be able to pay the money back. They just requested the documents and said the refund will take place in 7 business day. I need that money paid back.