Does the land sale broker deserve the agreed-upon amount, despite knowing about the increase in the land's price for the buyer and not intervening when the land was trespassed upon?

The broker is entitled to the full agreed-upon commission upon completion of his work, and this is a debt owed by the buyer that must be paid; because his task is to bring the buyer and seller together to complete the sale, and he has no relation to preventing the owner's brother from tampering with the land.