Loot, the Anomaly

If another effect causes Loot to deal combat damage equal to his toughness rather than his power, use his toughness to determine how much combat damage to assign, even if he also has negative power.

Loot's first ability doesn't change his power. It changes only the amount of combat damage he assigns. All other rules and effects that check his power use the real value, even if they cause damage "equal to a creature's power" to be dealt. In that case, if Loot's power is negative, no damage is dealt.