No crime. If you and I agree to some sort of payoff in the future, and you pay me for it, but we don't agree on what it is (it's a "favor to be named later"), nope. Otherwise, virtually every large contributor, lobbyist, etc. belongs in the can, especially the guy who raises $1,000,000 for a candidate and ends up Ambassador to Luxembourg. Again, Bonasera asked the Godfather to commit the crime, and the Godfather simply financed the payment until Sonny got killed, something someone in your field should recognize (though it would be tough to securitize the right to a really good embalming).
(edited) John, you're right, the Supremes did it in 1994 for some crimes. Wow, been a while since I been in law school. Doesn't change it though. Also shows how rotten federal criminal law is. One of two Scalia decisions I can recall agreeing with is when he said that the RICO statute was unconstitutional (flag burning was the other one).