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?
I have an ok lifestyle with the people in my house all working now, so would put all of it to use, that’s fine. If I am allowed to keep this in my community and city, I would put it in a trust that could only be used to extend the trolley system here back to at least what it was in the 1940s, and as much farther as practical with that $ and make it free to ride. An endowment that could generate enough money to keep that going and to modernize it.
If it must be global, how many people are there? 8,9 billion ? That’s not even a dollar each, I’m not sure. Maybe plant 5 billion trees, or buy up land & rewild it.