Instead of using mana, it might make more thematic sense if it uses gold. Especially since it's more like the merchant is hiring thugs instead of summoning them. Perhaps after you gain this ability (at a certain level, perhaps? Or even from the beginning, as gold is gold), the game asks you how much gold you want to spend, and then you get a number of thugs dependent on how much gold you spend. Offer too little gold, and no one comes to help.
Additionally, instead of them just "appearing" spontaneously, they could actually be created outside of the PC's line of sight. That way, it's more like they're running in to help, and less like they're being magically summoned.