What is the ruling of Islamic law regarding my brother taking 100 Egyptian pounds monthly from the tenant without the knowledge of his friend, the company manager, in exchange for his mediation in renting the house?

Your brother is an agent for his friend in renting out the house, and he is not entitled to increase the rent to obtain money for himself, as this is a betrayal of the principal. He is allowed to request a fee for his effort in searching. It is not permissible to take a fee for the guarantee, because it is a contract of gratuitous assistance (irfaq), not compensation (mu'awadah). The guarantee in this case would be conditional on transgression or negligence, because the house is a trust (amanah) in the hands of the tenant.