Matrix Warehouse Benoni

I purchased a router from Matrix Benoni - as I have been having issues with my Internet Service Provider and Fibre. Not one of the sales people mentions that you aren’t allowed to return any goods purchased. So back to my story I didn’t use the router and returned it the very next morning in the original packaging and sealed and I was told that they won’t give me a refund only an instore credit which I must use within 30 days or it expires, so I have given them the unit back and they have my money (is that not theft) They said that I returned the router due to buyers remorse they’ll only refund if the unit is damaged or not working. Sorry but I am not understanding how this is possible, I don’t need anything in the store but I am forced to buy something within 30 days?