Debit orders without permission

I have a personal loan with Nedbank and made an arrangement with you to pay it off monthly. I have been paying every month, and even received sms's from Nedbank thanking me for my payment and the arrangement made. My Debit orders went off on the 1st of December as arranged. Then on the 7th of December, another debit order went off for R1417.37! This is not even my instalment! I requested a refund: reference number: ***. Lady said my refund will be made within 7 working days. Today is the 7th working day, and nothing in my account. I phoned in again today, only to hear my request has been declined?? I ran out of airtime and asked the lady to phone me back, but alas. No call. I can not understand how Nedbank can just take money from my account without my permission? Not even a notification? This is not the first time that it happened! I have to live on a strict budget to make all ends meet, and you just take my last money in my account. This is not acceptable seeing that I have an arrangement with you! I will not leave it here!