Is the commission of a broker permissible if he bought a plot of land for 270,0, on condition that he takes a 20,0 commission, then refused to introduce the seller to the buyer or arrange a meeting between them, which raised suspicions about whether he was the buyer himself or intended to sell it at a higher price?

The broker does not deserve the commission unless he completes his work. If he buys for himself without the seller's permission, he deserves nothing, because an agent is prohibited from buying what he is appointed to sell. Furthermore, a broker selling for more than what the owner was informed of is deception and consuming wealth unlawfully, and the excess he takes is illicit and impermissible.