An offer to prove a general custom among brokers acting for both parties to an exchange of lands to charge commissions to each, held properly refused, for the reason that it appeared that the broker was the agent of one of the parties, and could not, therefore, legally demand compensation from the other. Dartt v. Somnesym, 86 Minn. 55, 90 N. W. 115. See also Sec. 578.