The funny thing is this is a decent enough Addams Family, and he'd likely have little to no trouble recouping all his money (maybe less shipping) on a local sale in Canada.
This is not your problem OP. All this idiocy about mods, some other game he found blahblahblah...not your problem. You made a HUGE mistake initially agreeing to accept the game back less shipping - probably added a week or so of nonsense from this guy. But when he realized you were receptive he got greedy.
Send him a terse, SHORT email stating that you never agreed to take the game back if he didn't like it, that you consider the sale final and that's that. There will be some angry emails and maybe some phone calls (which you should ignore and of course block if possible) and he'll go away.