Nothing in this chapter shall preclude any employer from deducting from the wages of the employees and paying over to any labor organization, or its authorized representative, membership dues in a labor organization; provided, that the employer has received from each employee on whose account such deductions are made, a written assignment which shall not be irrevocable for a period of more than one year, or beyond the termination date of any applicable collective agreement or assignment, whichever occurs sooner.