Contract dispute

2 years and 3 months ago I took out a 24month contract at MTN N1 city,when I enquired in January as the contract should have been closed off in December when last payment is due they informed that I actually signed a 36 month contract. I then took them the signed 24 month contract and even liased with the store manager Anthony Louw via email and sent the picture as proof. Still they keep deducting as he agreed that this was not right verbally but finance department decided that the fine print says 36 months,when I enquired about the employee who signed me up for 24 months as we both signed and she wrote 24 months, I was just informed she doesn't work there. My initial payment was 576 now it's soma 700. I distinctively remember that I did not want 36 months as I had enquired other service providers like Telkom and Cell c....This sales lady informed I was lucky as this was the last of their black friday promotion. Since February I have been reversing payments and tried to stop Debits as they could clearly see there was written 24 months. I have escalated this problem to their complaints office but seems like nobody is able to assist. Luckily I kept the contract as proof,could I please get some assistance in this regard.