Vodacom Telesales Misrepresentation & billing

In January I got a call from Vodacom saying they are changing my 5 gig contract to a Ten gig with a new black wifi at the same price of R199 as they are loosing customers to other network. After delivery my account was debited & I stopped Vodacom debit order. Asking why they said sales person misrepresented a sale and offered me a new contract. Media said my fee now is reduced to R179, buy billing is now giving me a R900 bill with unknown debit order stopping charging from my original R410 bill. I can't afford the R900 bill & see that now Vodacom is forcing sales to unsuspecting customers instead of the R600 expected after my enquiry. Vodacom cannot represent telesales contract and charge unknown fees. I insist on my 10 gig contract as I never asked for it & tired of sms's saying I should pay outside my payment cycle the stop debit order. They should cancel the 5 gig contract and leave me alone. Thanks