I don't think it would be that big of an issue. Williams needs the money so he should have no problem signing another legal document that says he has to exercise the player option and return to the Dolphins.
I'm sure the Argos and the rest of the CFL owners want this situation wrapped up soon. Williams is going to sell a ton of tickets. I bet you at least 40,000 show to the Argos home opener if Williams signs with the team.
I'm curious to see how he performs. I don't remember the last time a Pro Bowl calibre player in his prime left the NFL to play in the CFL.