I want to Exchange My Pants

So I bought a pair of pants amongst other things for my brother at Truworths man, cash (not debit card but real money) when I got home the pants didn't fit him but because I had been out shopping the whole day at Gateway I cant find my slip. the pants still have all the tags are still in the original Truworths man packet I decide to return the like I would at woolies, woolies would give me not problem but scan the pants and exchange it hassle free, however here I was told BECAUSE I DIDNT BUY ON ACCOUNT AND I LOST MY SLIP I COULD NOT EVER EXCHANGE THOSE PANTS. Because I bought with cash I could not EVER get something that I want. here I am stuck with a pair of hemisphere jeans. According to the CPA which your store manger has no Idea what it is, it is the onus of the seller to find my proof of purchase if I have lost it according to the south African law I am allowed to exchange anything you need to find the slip in your records and that is why woolies and any other shop including Mr Price never ever give you a hassle with an exchange if you have lost your proof of purchase if the item still has tags they scan it back into stock.