Exchange of goods

Bought a soda stream 60L gas Cylinder on the 9th of September at Pick ‘n Pay The Glen when I got home to use to the product, I then realise that I was given the wrong ***** on and it does not work on my machine, on the 10th of September I then returned to the same Pick ‘n Pay for an exchange and I was told they wouldn’t help me because we had removed the plastic on top of the ***** (You can’t tell the difference in the *****s when it’s sealed) we still have the slip and the cylinder hasn’t been used