YASID (Not "Button, Button")

I'm trying to remember the title of a story where a mysterious
figure offers people money for pu****ng a button that will kill a stranger.
At the end, the button gets pushed, the money is handed over and then
the figure sets out to hand the button to someone who doesn't know
the protagonist, the implication being that the protagonist will be 
the next victim.

	It's not "Button, Button" or if it is, it's a varient version
of it.
-- 
http://www.livejournal.com/users/james_nicoll
http://www.cafepress.com/jdnicoll
(For all your "The problem with
defending the English language [...]" T-****rt, cup and tote-bag needs)