What is the ruling on working inside a chicken slaughterhouse, knowing that its meat is unlawful?

It is not permissible to work with a slaughterhouse that does not slaughter chickens according to Islamic law, because that involves cooperation in selling the meat of carrion, which is forbidden. Allah Almighty says: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression). The doors to lawful provision are many, and Allah provides for whoever fears Him from where he does not expect.