MTN South Africa making me pay for their lies they sold me

A guy called me and told me that I can upgrade and get 50G anytime and I agreed because I was only paying R200. Now the data is in and they only gave me 20G and some useless promotion 10G. Now I want to cancel because this is not what I ordered and MTN says I must pay the cancelation fee. This is BS because I did not want 20G. MTN **** so I must pay for their lies? No man I hate this network and regret taking a contract with them