Their best bet is to see those adventurers die in the dungeon, hide the loot somewhere, and keep saving. When they have enough loot, they can put a price on the slaver's head, and watch as she gets killed. It's obvious they can't buy their freedom, the money would be just robbed from them and that's it. Trying to hire someone directly to be an assassin could also be risky since they have no presence in the city. They would just be asking to get robbed again. However, if it's a price on the slaver's head, there would be no end of shady people looking for quick gold. They wouldn't get the best people, but I doubt they would need to.
Of course they could also randomly meet some blessedly benevolent mage who could remove the slave imprint.