wrongfully DEBITED R1412.75,

I defaulted my November payment and on the 18th of December 2015 I made payment of R700 to MTN at the ABSA ATM and R712.75 was outstanding to MTN. on the 28th of December 2015 I was debited R1412.75,i called them on the 29th to enquire why they debited my account double the amount meanwhile I made payment and I was told I have credit of R700(due to me) and I requested a refund immediately and I was told I would be refunded with 2 to 48 hours, I called again only to find out I would be refunded within 2 to 7 working days, I must have heard incorrectly I guess, I then gave MTN a call again and I demanded to be transferred to a manager and ask him to refund me immediately he told me banking wanted proof of payment but I don't need to provide proof of payment cos the system shows I have credit. He then emailed me requesting proof of payment I told him I have misplaced it and I don't understand why I need to provide it meanwhile the system shows I have credit? I asked him to provide screen dump to banking as proof of payment and he said he will but to this day no refund was done. I don't understand who MTN works 10 days later their system is not updated to show payment has be made?