Following things might be absolut BS, so be prepared: If the legal status of an US foundation is comparable to a german "eigetragener Verein" (what KDE 'e.V.' is), they have to spend the money relatively quickly to drop back to a certain amount of operational costs.
This makes it relatively hard to plan if you have some years with huge income followed by years with smaller, but you cant slow down spending the money because of your legal form.