Xander's Pact

If you pay the casualty cost of a spell, the copy will resolve before the original spell.

If you cast a spell for another cost “rather than pay its mana cost,” you can't choose to cast it for any alternative costs such as a blitz cost. You can, however, pay additional costs, such as a casualty cost. If the spell has any mandatory additional costs, those must be paid to cast it.

The copy of the spell is created on the stack, so it's not “cast.” Abilities that trigger when a player casts a spell won't trigger.