You get to keep only enough to maintain a very modest lifestyle in a low-cost-of-living area, the rest of it has to go towards improving the world in some way.
Edit: Given the previous rules that you must maintain a very modest lifestyle in a low-cost-of-living area, would you rather choose to opt out and not have the money at all?
Buy the US House and Senate.
$5 B / 435 = 11,494,252 per person. Sounds do-able. Shit, Bob Menendez sold out for $480,000. 11 mil. would go a LOOONG way.