In an action for a real estate broker's commissions, parol evidence that the written agreement of prospective purchasers with the owner was not finally binding, and that after it was revoked, plaintiff sought to procure new purchasers, is admissible on behalf of such owner to rebut the effect of such agreement in evidence. Folinsbee v. Sawyer, 157 N. Y. 196, 51 N. E. 994; Buttz v. Cook, 113 P. 282, 62 Wash. 90.