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?
$5b wouldn’t do much in the grand scheme of things but it would make me the wealthiest person in my city 10x over. I’d fund progressive campaigns across the board to stack the legislate with like-minded people and then work on building my local community and hope that it has leeching effects on the surrounding area, state(s), country.
I feel like we have very different beliefs on what “grand scheme of things” means because I don’t believe $5b will make an immediate difference right away for most people, but you can implement small things that will create ripple effects that greatly change the grand scheme of things for the future.