contract upgrade installment debitted in correctly

I hav a contract did an upgrade, my previous installment was R388 open line nd I upgraded to tablet 10 inch nd was advise my installment is R275 I greed after my upgrade come to the debit date was debited more than what I was advised so I just want to cancel take yo tablet nd end my contract I won't pay any settlement, nd spoke to this useless so called supervisor \VUSI"in the contact centre telling me another useless excuse my money back or end contract I'm tired of cell c"""