Takealot not replacing faulty item

I bought a gas heater on Takealot and upon trying to use it, I discovered that it is faulty. I logged a return and it was booked in and collected. Now Takealot doesn’t want to replace the heater, they are offering to give me credit or refund because they are saying there is now a price increase on the heater. They are saying I bought it while it was on sale and now it is back to its normal price. The heater is still under warranty and I am not going to accept a refund or a credit. I want another heater.