The 10% stacks additively, each time you do the deal, this status is applied to a different 10%. If you sacrifice 10 fingers, the entire world is guaranteed basic human needs (food, water, shelter, clothing, healthcate, etc…)
Nobody knows you saved them. Would you do it?
If everyone knows about the fact that you can do this deal, would that change your mind?
How are they taking the fingers. Do they just disappear or is it something gruesome like having them chopped off? Blood is icky.