"Where, in an action for a broker's commissions for negotiating a purchase which defendant refused to consummate, there was testimony for plaintiff that defendant had demanded the return of a writing executed September 10, alleged to show a contract of employment, at the same time recognizing his obligation to pay plaintiff a commission, defendant should have been permitted to show that the paper he demanded referred to another transaction, and that at the same time plaintiff presented a statement for other commissions, not including the one sued on, and did not, prior to October 20, make demand for the sum claimed. Id.