What is the ruling on the agent's fees and his efforts, paid by the tenant, in the buying and selling transaction that took place between the heirs and the tenant?

The agent works for the benefit of his principal, so it is not permissible for him to take money from the other party without the knowledge of his principal; otherwise, it would be unlawful (haram). It is permissible for the agent to take a commission from both parties with their knowledge. The agent must inform his principal of any amount he took from the other party in excess of what was declared. If the principal permits it, then that is fine; otherwise, the original ruling is that it belongs to the principal.