You can and SHOULD trade the damsel back to the fish store - IF they will take him. But don't expect more than a dollar or maybe two bucks for him. They certainly aren't going to give you $5 for him.
Or, you can give him away to a local reefer. The only thing with giving him away to a local reefer, is some people will feed him to a big lion fish or eel. When I gave away my 3 stripe damsel, I asked the guy if I could come over and see the tank to be sure it wasn't a 200g predator tank. It wasn't, and I gave him the fish.
I know it sounds horrible (it does to me), but there are actually people out there who will feed them to lions or eels. :grumble: