Refuse to repair 3 month old device

We took out a contract with MTN about 3 months ago. For a month now we have been struggling with the device speaker and we finally got a chance to get the device back to MTN. Upon arrival, the clerk tells me that the will not replace the device due to a mark on the cover. She advised me to take the device to their prefferes cellular store to have it repaired. I handed the device in, only to receive a quote of R3400 to repair the device. I phoned the store and the clerk adviced that the bill is for my own account as there is a mark on the cover and voids my warranty. Now I had no problem with them not replacing the device due to that 1 mark, but now I am liable for the repair cost as well?? Which means that for 21 Months we will need to pay the contract and still get another device as the original device's speaker is ******ed

I think this is pathetic and daylight robery