Where defendants gave plaintiff an option to effect a sale of coal property, if sold within a certain time, on a stipulated commission, and agreed to assist plaintiff in the sale thereof, defendants will not be heard to say that a sale to one with whom plaintiff was negotiating, made during the continuance of the option, was the result of their independent efforts. Wells v. Hocking Valley Coal Co., 137 Iowa, 526, 114 N. W. 1076.