So if you fail you donate nothing?
How about committing to $1,000 donation if you stay the same. Then deduct a prorata amount for every pound you lose to get you to the $200 donation that you would prefer to make by Jan 1.
I don't know that I would make it too self-executing. Lest martinfocazio find himself chained to a radiator in a dark basement with nothing to satisfy his hunger but a canteen full of tepid water.