With the snowfall from the mountainside and access to a caravan or rail line, the Blackfoots can become water sellers. You need to convince the leader of the Blackfoots and the leader of the Hangdogs to to agree to this arrangement.
CombatBoy - Can threaten to murder everyone if they don't start selling water.
DiploBoy can convince people of the wrongness of slavery and offer water as an alternative commodity.
Stealth Boy - Could free the slaves, or poison them, eventually getting rid of all of the slaves. Then, as DiploBoy-light, tell them that angry spirits have entered the town and are unhappy with the slaving, but the spirits would be mollified by the Blackfoots trading in something else, such as water.
The Blackfoot tribals have access to a lot of water, mainly from rain and snow that falls on their village. If I could convince them to sell water they could help the wasteland and profit themselves at the same time.
I coerced the Blackfoots into starting caravans to sell water to other settlements in the wasteland.
I talked the Blackfoots into starting caravans to sell water to other settlements in the wasteland.
I coerced the Blackfoots into starting caravans to sell water to other settlements in the wasteland, and even convinced them to stop slaving!
By helping the slaves escape, I tricked the Blackfoots into thinking that the spirits were upset about the Blackfoots being slavers, but would be mollified if the Blackfoots traded water instead.