When he dropped it off, did he leave food? If not, what has it been eating all this time? Give it back to the owner, or better yet, make it simply disappear. I don't know where you are at, but here in TX, there are plenty of coyotes, buzzards(sp) and other scavengers that would make a dog carcass disappear in a matter of days. Or you could even bury it. Next time the owner shows up and asks about it, you haven't seen it in a few weeks and thought he came and picked him up. Sounds kind of heartless, but from what you describe the dog would be better off dead.