Migration done under false representation

I was advised by Nonthando at the MTN the Grove in Nelspruit that I can migrate my current 10GB to the 20GB package with a monthly cost of only R499. This was confirmed many times by the agent from the 21st of September. However, on the 25th of September I noticed that my billing will now be R1045 per month after which I contacted Nonthando again and she still claimed that the fee will be R499. After insisting that she confirm this she advised me she made a mistake and will rectify this by changing me back to my original 10GB package. Days went by and after fruitless hours on the phone with the call agents on 808, I was advised that only she MTN outlet can deal with this. I logged multiple queries online which still has received no response, posting in Facebook only resulted being referred back to calling 808 or sending message which I have already done but without response. After sending multiple emails to customer care and demanding a response I only received a reply saying that the cost of R1045 for the 20GB package is correct and I can only migrate after 3 months. So nobody actually is willing to admit the mistake and resolve. I need a way to have MTN admit to their mistake and rectify is ASAP