Proof that a party has executed a formal contract to convey certain property in exchange for other property is sufficient prima facie evidence of his title thereto, in an action by a broker for commissions on effecting an exchange. Muscowitz v. Hornberger, 46 N. Y. S. 462, 20 Misc. 558. Compare Herscher v. Wills, 103 I11. App. 418.