Unauthorized amount of deduction from my acc account for debit order

I went to find out about my existing contract expiring date n if I can take I laptop that I saw advertised on their magazine n the consultant who assisted me at MTN Goldfield Mall at Welkom in Freestate told me that it will expire end of June month but if I want to take something it is allow, that means I can take the laptop and start paying for this new things when my contract expires but I'm free to take the goods that was the begging of May, only to find when the debit order runs through the my bank the new amount for the goods or laptop that I took is now added on my monthly payment n of which it was not something that I agreed upon of signed for I even checked my contract, I call the consultant to find out what all that n he even showed me that contract n told my that I have to phone the call center cos on his side it's what I have signed for, I called the call center n they told me that the consultant who assisted me is the one who made a mistake canceled my contract before expiring date n called the consultant he said no he didn't do that so I have been send from pillar to post with no problem being solved I need someone to help me with this matter with MTN I believe I have not been treated fairly they have no right to deduct the money that I did not authorize from my account.