Oracle Text

Sacrifice a white creature: For each attacking red creature, prevent all combat damage that would be dealt by that creature this turn unless its controller pays 2R.

Card Rulings

10/4/2004 The cost to avoid the penalty must be paid when the ability resolves. You can’t wait until later to do so.
10/4/2004 Paying the Heroism cost to allow a creature to deal damage does not prevent other effects from preventing it from dealing damage. Paying the cost only prevents the Heroism effect.