MTN is a ****

I had five lines with MTN of which only 1 of them is remaining because I am trying to leave MTN because of the very dodgey experience they gave me throughout my relationship with them. My remaining line is a data line that I am supposed to pay R400 for. Last month I tried to upgrade to a better deal but the consultant I was talking to misled me so they reverted me back to my old contract. Regardless of that not being my fault I was told that I still owe pro rata and my bill amounted up to 823 and that was fine. I paid the 823 at the store not aware that the debit order was still active so they debited 982 out of my account. I tried to get 982 today and they are telling me that they will not be refunding me because conveniently I now owe them 1823 and I requested an invoice which I have not received.