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?
In the US, so free health insurance for those making under 100k for 6 months (or likely less - whenever the money runs out). Maybe that would give enough people a taste of universal healthcare that they would start voting for policies that get us closer to that.