@omniRex : They did actually, that IS why the boss put the bounty on the slave trader and 'the girl with him', in fact these guys came specifically for the slave trader.
See these two pages:
https://mangadex.org/chapter/1130352/15
https://mangadex.org/chapter/1130352/18
Basically, they heard the slave trader made some money and so went to bully him to get some part of the debt he owed them back. (ch 6)
After hearing about the monster, the boss assume that the slave trader decide to fight against them (because he was unharmed in all that) and put a bounty on him.