Hi,
I'm afraid I don't know the legislation in your country but over here stores will basically lie to your face and say they cannot refund money only offer credit or replacements.
Once you quote the law however they usually have the sense to give in.
But I would make it a priority to find that reciept as they may try and claim a time limit on the problem.
Look up the laws relating to faulty goods ( which is sadly what an already sick animal falls under in most cases ) and be prepared to be polite but firm if they are not complying with it.
But get your paperwork so you can prove what you bought and when if push comes to shove.
dr del