Keep it. Uneducated employees are one of many money losses in the fish industry. If it would have died in the store, they would have been out wholesale cost as well as what you paid. If you need peace of mind, call the store and tell them it died, they can't prove otherwise.
BTW Oddball, are angelicus synodontis really $100? I bought a 4" one that was the gold spot variant from a fish manager at a petstore because he said he didn't like those kind of fish and the previous manager ordered it. I paid his created price of the top of his head of $13.