mtn billing incorrectly

In July a mtn consultant phoned me to upgrade one of my contracts. the deal was R199 x 24 months for 10GB night/day time. now as from end of july they are billing me R697.95. I phoned many times and also spoke to the guy who did the contract . and still not sorted out. I wont pay may account at all until this matter is resolved I have got the proof that the contract was for R199 .