Is taking a commission exceeding 5% on apartment rentals—even though the law sets it at 5%, but it's done by mutual agreement between the office and the tenant—considered lawful money devoid of blessing?

There is no harm in taking a commission for the effort of renting apartments and the like; this is called brokerage. It is stipulated that the commission must be known. It is not permissible for the commission to be an unknown percentage of the rental amount, according to the majority of scholars. As for the state's setting the commission at 5%, this falls under the ruler's restriction of what is permissible for the public interest.