This is a modern phenomenon which has never quite clicked with me. If, along with a few dozen, hundred, thousand, others you agree to hand over £x to some athlete prepared to run 6 marathons back to back or ride up Mount Kilimanjaro on a unicycle, and you are completely trusting that, if he succeeds a large proportion of this £x will be donated to a favourite charity, would you, in the event of him dropping dead or otherwise failing in his mission say. "Oh no, sorry you can't have the money"?
If your answer is "no", why not hand over the whole sum right at the start and save him the trouble. Tell him you want the whole sum to go straight to the charity and he should fund his adventure some other way.
Just a thought . . .