Where the correspondence by which plaintiffs were employed to sell land does not cover the question of compensation, a former oral agreement as to compensation to be charged may be shown, but where the contract by correspondence refers to a sale of lots at specified prices and terms, evidence of a former oral agreement that the sale should be made subject to' defendant's approval, and that the deed should contain certain conditions, is inadmissible. Sayre v. Wilson, 86 Ala. 151, 5 S. 157.