A real estate broker is entitled to the commissions agreed on for the successful negotiation of an exchange of property placed in his hands, if the terms of the exchange are accepted by the owner, as the obligation to pay the commission then becomes fixed. Lockwood v. Halsey, 41 Kan. 166, 21 P. 98; Einhorn v. P. Derby & Co., 132 N. Y. Sup. 327; Clark v. Asbury, 134 S. W. 286, - Tex. Civ. Ap. -; Caldwell v. Tanehill, 84 S. E. 6, 117 Va. 11. In the absence of a provision to the contrary in the contract of employment. Frye v. Schwarz, 87 N. Y. App. Div. 611. See also Secs. 498, 499a, 499b.