How long had you had the puffer? If he has been in there for a while and he's acting like that, i would say it would be a good idea to give him back if the LFS
would take him. However, if it had only been a couple days I would not have been as concerned because it takes some fish a long time to acclimate to a new tank.
That being said, if you had inverts in the tank and wanted to keep a puffer, you would have seen all your inverts eaten eventually. So it was probably a good idea to give him back regardless of his condition.