A contract, where the owner of real estate, in order to induce an agent to accept a stipulated sum for his services in effecting an exchange, agreed to pay him more if the sale proved satisfactory, could be established by parol evidence as an independent agreement, though the stipulation for payment of the first sum was in writing. Blair v. Slosson, 27 Tex. Civ. App. 403, 66 S. W. 112; Bradley v. Bower (Neb. Sup. '04), 99 N. W. 490.