Is it permissible to buy land directly from its owner after he rejected a previous grace period from the buyer, knowing that the broker was the one who first introduced the buyer to the land?

There is no objection to purchasing real estate directly from its owner. The broker is entitled to a commission if the custom of sale includes their role in introducing you to the property, even if the sale is later concluded directly through the owner. In this regard, reference is made to custom to determine the entitlement to the commission and its amount.