Heroism
Enchantment
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 
.
Mark Poole
Fallen Empires (FEM)
• #6Rulings
The cost to avoid the penalty must be paid when the ability resolves. You can’t wait until later to do so.
(wotc • 2004-10-04)
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.
(wotc • 2004-10-04)
Rulings
The cost to avoid the penalty must be paid when the ability resolves. You can’t wait until later to do so.
(wotc • 2004-10-04)
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.
(wotc • 2004-10-04)