MTN South Africa taking advantage of the disadvantaged.

I have bought Samsung Z flip 4 from MTN South Africa ( Thavhani Mall in Thohoyandou). The phone developed overheating and started with green and pink lines on the screen. I took it back to the store and agreed to take it for repairs but now they are saying I should pay for the repairs and the phone is still under warranty and has no physical damage. I have heard so many people experiencing the same issue as me, which clearly indicates that MTN or Samsung must take acccountability for this and i never stooped paying for the phone. Anyone help