A broker employed to effect an exchange of land ordinarily becomes entitled to a commission upon the execution of a contract therefor. Blaydos v. Adams, 35 Mo. App. 526; Shanks v. Michael, 4 Cal. App. 553, 88 P. 596; Rohkohl v. Sussman, 113 N. Y. Sup. 586, 61 Misc. Rep. 246; Carrington v. Smithers, 147 P. 225, 26 Cal. App. 460; Keinath, Schuster & Hudson v. Reed, 137 P. 841, 18 N. M. 358; Carroll v. Lafgreen, 170 I11. App. 328; Mercantile Trust Co. v. Johnson, 160 S. W. 535, 177 Mo. App. 503; Duncan v. Turner, 154 S. W. 816, 171 Mo. App. 661; Jennings v. Overholt, 172 S. W. 440, 186 Mo. App. 505; Deweese v. Brown, 135 P. 800, 55 Colo. 430; Jauman v. McCusick, 137 P. 254, 166 Cal. 517. See also Sec. 186.