The burden of showing that one employing a broker to find a customer to exchange real estate with him had notice that the broker was to receive a commission from the customer is on the broker. Hannan v. Prentis, 124 Mich. 417, 83 N. W. 102; Robinson v. Clock, 55 N. Y. S. 976, 38 App. Div. 67; Stapp v. Godfrey, 139 N. W. 158, 158 Iowa, 376; Thomas v. Wychoff, 174 N W. 26, - Iowa Sup. - . Compare Redmond v. Herke, 137 Iowa, 228, 114 N. W. 885.