Where a contract specifically authorizes the sale of a tract of land containing a certain number of acres, and it does not appear that prior to the sale the owner claimed, or that the agent knew, that there was a greater number of acres in the tract, evidence that the tract did contain a greater number of acres is inadmissible to defeat an action for commissions. Howell v. Denton (Tex Civ. App. '08), 113 S. W. 314.