broker's right to commissions

a broker is never entitled to commissions for unsuccessful efforts. ... If, after the broker has been allowed a reasonable time within which to produce a buyer and effect a sale, he has failed to do so, and the seller in good faith and fairly has terminated the agency and sought other assistance by the aid of which a sale is consummated, it does not give the original broker a right to commissions because the purchaser is one whom he introduced, and the final sale is in some degree aided or helped forward by his previous unsuccessful efforts.