Is the money that a relative earns from renting residential apartments in a company in a non-Islamic country considered usury (riba), or is it an ordinary, permissible (halal) commission?

Your relative works as a real estate broker for apartment rentals. Brokerage is permissible as long as it does not involve assisting in something forbidden. If his fee is a fixed amount for each apartment he rents out, then this is a valid ju'alah (stipulated payment) with no objection. However, if his benefit is determined as a percentage of the rental amount, the majority of scholars believe that setting the wage as a percentage of profit invalidates the ijarah (lease/hire contract) due to the presence of gharar (excessive uncertainty), citing the Prophet's (peace be upon him) prohibition of gharar and of hiring a worker until his wage is made clear to him. Some scholars, however, have permitted the wage to be a percentage of the profit, citing examples from the Salaf (early Muslims).