They don't want to refund

On the 19/2/15.I took my Toshiba laptop charger with me to Dion Wired for a new 1, as I believed that it was not working. I showed the salesman the charger, and he gave me a universal charger. I paid for it and left, when I came home I took the charger from the casing and connected it to the laptop, the blue light on the charger went on, but nothing happened with the laptop. I immediately though that it was at this point the laptop or the battery seeing that the charger was working, I discard the casing, cause I didn't see any reason keeping it. A family friend came to have a look, and saw that the charger is not compatible with the battery of the laptop. My son returned to the store with the receipt and charger, I didn't have the casing because I discarded it, not knowing that the charger will not work on my laptop. They refused to refund, say they need the casing, I called this morning and spoke with the same manager that my son saw on 25 Feb, he gave me the same story that they cannot refund without the casing, why am I the customer punished when the salesman didn't do his part. My son took the laptop with to show them that it was the wrong charger, but they still don't refund