Disappointing service

I have a contract with MTN. Last year in January I wanted to pay off my contract so I did my own calculations and I got R3600. I called their customer service and I was advised to call the retention department so that I can have the correct amount. I did as I was advised to do and spoke to Kgosi from retention and he told me to pay R3619. I went to their store in Carlton center and paid R3620. To my surprise this morning I received my statement and it says my outstanding amount is R307. I spoke to Nandipha from retention department and she told me I was suppose to pay R3900 instead of R3600. She told me she's gonna send my case to Kgosi's supervisor. How is that gonna help me because this in an inconvenience for me and I can not afford to pay R307. The reason why I paid up my contract was to avoid paying this much money when my contract ends with MTN. I am very disappointed with MTN and I want something to be done about this because the last thing I need is to be blacklisted because someone didn't do the job well.