iStore Canal Walk - Sales Consultant

I went to the iStore in Canal Walk to get an upgrade via MTN as I was instructed to go there because MTN did not have the phone I had requested in srock. I got to the store and I was assisted by a Keenan Brandeth, a sales onsultant. I told him that I was instructed by MTN to come to their store as MTN did not have the phone that I was looking in stock and that I was due for an early upgrade. We did the necessary procedures and I kept confirming with him that we doing an upgrade, he was also nodding back and confirming what I was asking him. In the middle of the month I received an invoice that was double the amount that I normally pay. I was soo shocked. I called MTN so many times requesting them for help. After all the up and down that I had done they finally confirmed that they can't do anything on their end, and that the only option I have is to go to the person who actually made the sale. I went to the iStore in Canal Walk and asked for Keenan Brandeth, and told him what MTN had told me. He recalled me from the day the upgrade was done and even told me the model of the the phone I took. He then looked through the system to see what happened, that's when he realised that instead of an upgrade he created a whole new contract that's the reason why my invoice was sooo high for the past two months. He then told me that he will get back to me the next day because he needs to call MTN and see what they can do from their end, but one of the parties will have to pay but that would ould not be me. I then left the store. The next day he did not call but instead I had to send him an email to remind him that he said he'd get back to me. Keenan Brandeth from the iStore in Canal Walk then called to tell me that he cannot do anything about the problem at hand because MTN won't change the contract to an upgrade but will keep it as new contract under my name. He then told me that there's nothing he can do about that mainly because he also does not recall whether I came for an upgrade or a new contract that day and the onus was also on me to see whether he was doing he's job correctly. I am now without wifi nor data and sitting with an excruciating bill because of Keenan Brandeth, the sales consultant, who works at the iStore in Canal Walk.