At the LFS i work at, we wouldn't give you anything for it. The best we would do is give you a discount on another one, but that's rare. If your LFS will do better, then go ahead and try. The worst they can say is no. Just be prepared for that because most LFS do not have any guarantees on their livestock especially saltwater. Clowns don't have to be in pairs. They can do just fine on their own. If you want a pair, I'd wait until the first one is done with QT before getting another. The risk of introducing yet another parasite or pathogen to the so-far OK clown is high and complicates the QT process.
The selfish part of me really wants to come out of this ordeal with two clownfish in my tank. If this one bites it overnight how bad would it be to get another, assume it has whatever this one has, and to throw it into the QT with the so-far OK clown?
Once again I sincerely appreciate all the help you are providing. I don't know what I would be doing now otherwise.