Unhappy customer

I'm so disappointed in MTN I went there to get a Huawei nova 7 and bongani the sales consultant at fourways mall offered me oppo 54 which I liked and first of July I got my first deduction which was 900 and I called in and asked why coz our agreement was 530 and I was told about the pro rata ,k I understood but again this month in August got deducted 830 again and I called in and spoke with the manager and she told me I'm on billing which I was not told about ,and when I speak with bongani he said I can reverse my money which I will and the manager said I can reverse but my credit score will be affected they actually wanted to shut me down,and again this oppo phone is not working proper which I was told I can't bring it back I have to pay 8000 if I want to cancel,first I'm beducted way more n second phone is not working proper and lastly I'm getting threats of being blacklisted for my own money ,I'm not happy with MTN at all I want my 290that they deducted back.