MTN refuses to refund me on an overcharged account

I was overcharged for a contract phone i took with Northgate MTN shop in December 2019 and up to date MTN wont refund me despite having proved that they indeed overcharged me on the billing and settlement value . I have gone into the store with my contract summary which proves that my account was tampered with after i departed and that resulted in an overcharge until i resorted to cancelling my contract. All efforts toget my money have proved fruitless despite MTN having acknowledged that they indeed overcharged me. Itumeleng the agent who assisted me when i purchased the phone promised tp have the issue resolved but up to date nothing has been done.

I am very disappointed at MTN