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?
With 5 billion, you can provide housing for 5k to 50k people, you don’t even need to charge rent
Good point, actually. It’s more money than I realised