I would take it back as well, they should not have even had it on the shelves. It will get thrown away so it is not like they would need to repackage it to re-sell it. Most of the time at least with human medications, when a product goes past its expiration date, the risk is loss of potency rather than it turning into something harmful (except tylenol, my pharmacy friend always said to not risk taking expired tylenol). I have used meds that have been a couple months past their expiration date without any problems but that's just me and it was something I had had lying around the house, not something just purchased.
So the bottom line would be an ineffective dose of supplement for your cham, and I would not hesitate to take it back.