Unhappy dstv customer

I was called by an agent to upgrade my decoder to an explora for an extra R99 per month. I was and currently on dstv compact plus. My monthly subscription was agreed to be R639 per month and we confirmed everything on the phone. Now I get an sms that my debit order is R988 and when I call its a mystery. I can't afford that and if I knew that I'll be charged that I was never gonna agree.