Is the money received by the tenant from another person to take his place in the apartment, without the company's knowledge, and stipulating a certain amount as a one-time "brokerage fee," halal, knowing that this amount is not related to the rent paid to the company?

There is no harm in you taking money in exchange for someone else replacing you in the property you wish to vacate, because it is either in return for your guiding the person to the property, or it is a "khalaw" fee, which is permissible in this case; because it is compensation for your waiving your right to the usufruct that you sold to the second tenant.