Am I entitled to claim a share of the commission on any future real estate business conducted between my original client and another broker whom I introduced to them, whether I stipulated this beforehand or not?

Brokerage (commercial mediation) is permissible in principle and is considered a form of permissible commission (ju'alah). The agent is entitled to his fee as soon as he performs the work, and if a group performs it, they share it equally.

If the role of the first intermediary was limited to guiding the second intermediary to the client, and then the second intermediary completed the remaining transactions with the client, then the first intermediary is not entitled to anything from the subsequent actions after the initial guidance, whether this was stipulated or not, because a condition is void if it is not based on work, as the Prophet, peace and blessings be upon him, said: "Whatever condition is not in the Book of Allah is void."

Profit is deserved through capital, work, or guarantee. If none of these three reasons exist, then profit is not deserved.