Absa Let me down *****

I requested to sign up for an adult matric course during the week was contacted by a guy by the name of Clarence, he advised me on the subjects and amount for the course I agreed. I was contacted the next day by someone to confirm my details and the amount paying every month of which was R750 of which I agreed over a period of 15 months, now I received a debit check from my bank saying the amount is R799 of which I never agreed to I tried calling multiple times and was advised on a call recording they were going to sort it out but will now they haven't neither does the guy take my calls anymore or responds to my messages even the email address provided I've sent multiple emails but no response as for my bank Absa, they approved the debi check without my concent as I never approved it on my app as I was told to do so, so who give the authority for Absa who's just my bank to give the permission when I didn't especially for the incorrect amount