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?
If I got billions of dollars at this point how could I not throw a little at the owls. I would feel guilty everyday I open my subscriptions.
It is a nice feeling to see it means something to all of you!