I think you greatly overestimate how much power a billion dollars provides...especially since the IRS would lay claim to that billion if he ever tried to return to the U.S. Why would they work out a deal when they could get their hands on the billion and have an example for future tax evaders?

Note: I use the term tax evader in this context because if Saverin attempts to return to the U.S. in the future (as a resident or citizen), it will be clear that his renunciation of citizenship was primarily for the purpose of evading taxes.