Bad services. Frustrated

Hello Peter, I've bought a fridge at Kempstar cash crusader. They gave me 6 month warranty. My fridge is still under warranty and it's not working. I took the fridge to the store. They suspect that is a gas that is finished. They now saying I must pay extra money to refill the Gas while my fridge is still under warranty. When I bought the fridge it's not explained to me that if the Gas finished while fridge still under warranty it will be my cost. I told them I can't pay as it's not explained. They say it's written on the back of receipt and I went to read my receipt it's not written. I left the fridge at the store and they said they will not fix it until I agree to pay. How should I pay while my fridge is under warranty. Please assist