Bad service from Randal and Donavan

I did an early upgrade in December 2017 from a phone to Laptop. When I did the upgrade I told Donavan I still want to receive the 50min that i was getting at the time. He then told me OK. Which in my mind tell me I will be getting the 50 mins. Now January comes and I only receive 1 gig data. Lost almost 100 minutes that was saved up on the sim. Now I called to have the problem resolved and now all of a sudden I need to pay for the 50 min. I refuses to pay! Randal the team leader/ supervisor in the Retention's department confirmed that Donavan said OK to the 50 min but refuses to fix the problem. Should I been told when doing the upgrade "Sir you need to pay extra for the 50 min per month" there would be no problem. I want MTN to sort out this problem. I don't care who's paying for the 50 mins. I'm not paying for someone else's negligence. It is clear that Randal is taking the part of his Sale person (Donavan) who is in the wrong. I need someone at MTN to sortout the problem or give me the details of the Ombudsman.