MTN not sticking to terms with my 24 month contract

I am currently in a 24 month contract with MTN of which I had to R89 for 4Gigs of data ( 2Gig during the day and 2Gig at night) I am currently experiencing problems with the debit order from they because I think for the past 6 months they have been debiting R117 instead of the R89. I did lodge a complaint against them they kept on sending me texts and emails stating that my issues has been resolved but come month end the debit is the same.

Please assist I am tired of bieng overcharged