Heraldo de la Concordia

If the exiled Aura has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the spell has any mandatory additional costs, you must pay those to cast the spell.

The value of X is calculated at the time Herald of Amity's last ability resolves.

You cast the spell while Herald of Amity's second ability is resolving and still on the stack. You can't wait to cast it later in the turn. Timing restrictions based on the card's types are ignored.