Why not surender your selfish soul to this generous act and do it.

Another rationale is if you cheat and are found out, the karma will be worse than if you refused.

Really buy in to the event. Get a pledge sheet. Canvas the office looking for donations. After you've squeezed as much money out of the office as you can, hit on your boss for a pledge. When he sees the pledges from others in the office, he'll feel like a real schmuck if he says "no".