Taking more than what they supposed to.

I had a loan with Nedbank and i got some money from a Aunt and decided to pay that money on the loan, the amount i got was just a little less then the money i owed them according to my last statement, then they deduct the full amount that month, then i phoned them to tell them they owe me, but they insist i still owe them. I then emailed them and explained my whole situation, until today no response. I phoned again, the guy made as if the line was breaking up and put the phone down in my ear. Now i' m really ****ed. i decided to go into my bank and cancel the debit order, but, somehow they managed to deduct the money " i owe them". This has gone too far. I want the money back that they owe me. The banks are so quick to phone people when they owe them or whatever but when the roles are reversed, you struggle to get your own money back. I never skipped a payment ever and always used NB but will never again. Over and above the high interest you get charged they still want to get more out of you.