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 always wanted to see if I could ‘fix’ the Berkley Pit mine. It’s a superfund site with some of the most acidic water in the world. It was a cooper mine for decade that went bust. When the owners walked away, it started filling up with rain water. But, because of the way mines work, that water became VERY acidic. So now there’s this lake of acid out in Montana that no one wants to deal with.
Wikipedia: https://en.wikipedia.org/wiki/Berkeley_Pit
Inter-mountain Histories: https://www.intermountainhistories.org/items/show/376
ChemAnalyst News - Pit might have rare earth minerals: https://www.chemanalyst.com/NewsAndDeals/NewsDetails/montana-toxic-legacy-could-become-america-rare-earth-savior-36626