A broker must show that his acts were the producing cause of the sale, and it is not sufficient to show that his act was merely one of a chain of causes producing the sale. F. H. & C. B. Ger-hardt R. E. Co. v. Marjonie R. E. Co., 129 S. W. 419, 144 Mo. App. 620; Crain v. Miles, 134 S. W. 52, 154 Mo. App. 338.