Morale vs. temptation...what would you do?

A couple of months ago my husband and I bought a rug from a major dept. store for $650. A week later we decided it didn't work in our living room so we called and cancelled the order and told them to not deliver it and to send us store credit instead. A week later it was sitting on my doorstep. I called the manager and complained and told them they have to come back and get their rug and please send our refund. Two days later we decided it could work in another room. I immediately called the manager and told him that we will just keep it after all. Just recently when I got the mail I saw that the store had accidentally sent us that store credit for the cost of the rug ($650) My husband says just keep it but I don't feel comfortable. What would you do??? It is so tempting but I feel it is wrong. Should we keep it for a future purchase or call and return it?