Doing a Mace

To pull out of a commitment at the latest possible opportunity, often to the severe detriment of other parties involved.

Person 1: "Dude, can I have a kidney?"
Person 2: "Sure"

On operating table:
Person 2:"Lol jk, no kidney for you"
Person 1: "You're Doing a Mace! Noooo"*Dies*