Is the money I received in exchange for interceding on someone's behalf with his employer, for the purpose of undertaking real estate projects, lawful (halal)?

This work is considered brokerage (simsarah), and it is permissible to take a fee for it, provided that it is not a percentage of the profit but a known wage. If the contract of hire (ijarah (definition)) becomes invalid, you are entitled to a customary wage (ujrat al-mithl). You must not engage in anything other than what you were hired for, except with the employer's permission.