Well, not entirely. You can’t make an enforceable contract to sell yourself into slavery or order a hit on your ex-wife. I could see a court harrumphing about the bonuses being “against public policy” as they overturn them. However, that is a matter for a court and not the prerogative of the executive or legislative branches, if the rule of law is to survive.
Ah, but the bonuses can’t be against public policy if Congress provided for them to be paid. The Feds should just forget about the paid bonuses, and just make sure whoever is running the show for them knows what’s going on now. The Federal government owns 80% of AIG. They can do anything with it they want.