Where the real estate agents and the parties to the proposed exchange of properties understood that the agreement for the exchange, and any right to commissions, were dependent upon the defendants' acquiring outstanding interests in the property they proposed to exchange, and that their acceptance of the terms of the exchange offered by the other parties was, in fact, conditional on their acquiring such interests, commissions can not be recovered of defendants, their failure to acquire such interests not having been by their procurement or connivance. Rieger v. Merrill, 125 Mo. App. 541, 102 S. W. 1072. See also Secs. 45, 108.