No sale or assignment, by the head of a family, of wages, whether the same be exempt from execution or not, shall be of any validity whatever unless the same be evidenced by a written instrument, and if married, unless the husband and wife sign and acknowledge the same joint instrument before an officer authorized to take acknowledgments. Provided, however, that no such assignment or order shall be effective or binding upon the employer unless the employer has in writing agreed to accept and pay said assignment or order. This section shall not apply to a wage assignment by an employee to an organization which represents the employee in labor relations with the employee’s employer.