Under a written agreement of a land owner to pay a broker a certain sum if he would send or cause to be sent to the land owner a person with whom the latter "may see fit and proper to effect a sale or exchange" of the land, the broker can not recover the sum stipulated, without proof of a sale or exchange of the land; nor on a quantum meruit for services in negotiating for such a sale or exchange, without proof that such negotiations were rendered fruitless by the fault of the land owner. Walker v. Terrill, 101 Mass. 257.