How should one deal with a real estate broker who claims to have partners in order to secure a larger share for himself, thereby reducing the shares of other agents, while he actually works alone?

What this person is doing is unlawful (haram) because it involves two prohibitions:

First: Lying. Allah has commanded us to be with the truthful, and the Prophet, peace be upon him, warned us against lying because it leads to wickedness and brings one into the Fire.

Second: Unlawfully consuming people's wealth. Allah Almighty has prohibited consuming wealth unlawfully unless it is through trade by mutual consent. The Prophet, peace be upon him, warned against taking a Muslim's right without due cause and promised the Fire to those who do so.