It would require a transfer fee and maybe sitting out; depending on the current rule and how Mexico were to feel about things.
I know the US is making it tougher and tougher for guys to transfer. They are taking it seriously. Also allegedly barred RBY from a National team training camp that was held in PA a couple years back. Non- national team guys used to be able to go to these things in the past, bc training partners are needed. Not RBY anymore, or so the story went. If anyone knows the details, please include them or correct me.
If he did transfer back and repped the US, I'd be all-in on RBY; just like I was when the hated Drew Bledsoe ended up with the Buffalo Bills.