Magic: the Gathering (MTG) Enchantment card from Fallen Empires courtesy of Deckmaster
: Sacrifice a white creature to have attacking red creatures deal no damage during combat this turn. The attacking player may pay for an attacking creature to have it deal damage as normal.
FE • ENMark Poole
™ & © 1993—1994 Wizards of the Coast
Legal in: Legacy,Vintage,Commander
The cost to avoid the penalty must be paid when the spell resolves. You can't wait until later to do so.
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.