Problem doesnt get resolved

I did a upgrade on one of my contracts at langenbergmall Mosselbay. Samsung has a new thing if you trade in your old phone you get discount on your contract. I was super happy about it but now my problem comes in at MTN they still charge me R900 for the contract where I must pay R300 a month. Have been 3 times to MTN and phoned them still not resolved. So now I must pay so much on my contracts just to keep my line on.