Is the money that the man took considered lawful or unlawful, and must it be returned to the company owners after he bought an apartment with it?

What this man did, seeking a partner for the company and agreeing with its Christian owners to do so for a known fee, is called brokerage (simsarah), and it is permissible in Islamic law unless it involves assisting in a forbidden sale. Brokerage falls under the category of al-ja'l (a stipulated reward for a specific act).

Therefore, there is no sin upon this man in taking the aforementioned amount, and he is not required to return it to them, because it is a reward for the work he performed. The non-completion of the deal does not harm, unless the agreement was that the ja'l was for the completion of the deal, not for bringing in a participant.

As for his description of the Christians as "thieves," this is a form of sincere advice if the matter is indeed so.

Cashing the check before the due date is a mistake, but there is no harm in it if he feared their treachery.