Where plaintiffs claim they effected an exchange of defendant's property, and defendants assert that the trade was brought about by D., the question whether D. had received compensation for his services was properly excluded as immaterial. Creager v. Johnson, 114 Iowa, 249, 86 N. W. 275; Sewell v. Collison, 108 N. Y. S. 25, 123 App. Div. 586