What is the ruling on the money the questioner took from his colleague who was stealing from the restaurant, and is he obliged to return it, knowing that he does not possess it now?

It is not permissible to take from the wealth of a thief or an aggressor after knowing of their theft and aggression, for whoever does so is like a usurper. The taken wealth must be returned to its owner, even if through indirect means, or by intending to return it whenever possible; for the Prophet, peace and blessings be upon him, said: "On the hand is what it took until it returns it." It is not permissible to remain silent about the act of a thief; rather, it is obligatory to inform about him if it is possible without harm.