He hasn't actually lost 10B. He literally just moved it from the scrooge McDuck money vault to the "climate change" money vault. The fund, because it's an investment fund, will generate income to offset the small amounts he gives out per year, because it's not just going to be 10B in a day, it's going to be like "50m over 5 years for you, 100m over 4 years for you". He generates wealth from the fund, while simultaneously claiming on his taxes that he donated 10B to wipe out any potential taxes he would've had to pay
He literally gets richer by claiming this, and it takes years (if not decades) before 10B is actually given out to the causes.