********* agent that upgraded my contract

I agreed to an upgrade on my contract on the 06/05/2022 and it was over the phone with a guy named Lindani. The upgrade was to move to a data only deal of 28gigs which is 14 anytime data and 14 night time data for R99. The guy assured me that the remaining balance on my device subscription of R800 will be divided into two debit orders which I'll be debited R400 end of this month and next month. I was surprised to see this month's billing on the mtn app showing R919.94. I do not have that kind of money to pay mtn whereas that was not part of the agreement. Now when I call customer service I get referred from pillar to post. I wish I never agreed to the upgrade to begin with