Where a vendor of land instituted a suit against his agents, real estate brokers, to recover a part of the purchase money in their hands and retained by them as commissions, he was thereby estopped to allege the invalidity of the sale as against said brokers, and therefore it was immaterial to inquire whether such sale was or was not valid under the statute of frauds. Christensen v. Wooley, 41 Mo. App. 53.