Want my refund from cell c

Good afternoon I have a problem with cell.I paid my my contract I have with them.I paid R 500.And on the 2 of June they debited R390.I called the on the 2 of June about why they are debiting because I paid already.And they said I must send proof of payment and the bank statement.Because I want my refund And they said is taking 14 day for refund. But now they want the bank statement again It's like they don't want to give me my money..I am not happy with the service they are going me .Please help