Hi, I am having problems with MTN

Hi, I am having problems with MTN. I received a call from them and they offered me to migrate my contract to a data and minutes plan. I asked them how much will this migration be and how much I'll be paying they said it will be R290. I then agreed only to find out that now my contract is R349. When I call MTN they are now telling me its a third party called Infinity and they can not cancel the migration. I am not happy as I was mislead by the agent that called me and secondly I am also not happy that MTN gave these people my details as the lady already had my ID number and banking information. I want this to be fixed before I resume with my contact payments.