Does the broker deserve a commission on the purchase of land, even though he made no effort to find it, and even incited the seller to stick to the initial price to prevent it from being lowered?

Brokerage falls under the category of ju'alah (a contract for a specified reward for performing a task), and the broker is entitled to the commission if he performs the agreed-upon work. If he does not perform it, he is not entitled to anything. So, if your friend exerted effort in finalizing the contract between you and the seller, and thus became entitled to the ju'al (reward) for that, then he deserves it. His convincing you of the price does not affect this. However, if the ju'al was for mediation only, and he did not direct you to the aforementioned land, and he exerted effort on behalf of the landowner, not you, then his fee would be due from the landowner.