If someone uses Flaming Gambit against me for 5 damage
Can I choose to assign the damage to my Ember Hauler, pay 1 mana, and have it deal 2 damage to the other player?
If someone uses Flaming Gambit against me for 5 damage
Can I choose to assign the damage to my Ember Hauler, pay 1 mana, and have it deal 2 damage to the other player?
No. You only choose if you want to redirect Flaming Gambit's damage as it resolves, and you have to choose a creature you control at that time to redirect it to. If you sac the Ember Hauler before that point, it won't be around to redirect the damage to when Flaming Gambit resolves. If you wait until Flaming Gambit resolves, however, the Ember Hauler will be destroyed before you get a chance to activate its ability.
Fluttershy is best pony.