It shall be unlawful for any employer to contract with any labor organization and for any labor organization to contract with any employer for the deduction of any fee, assessment, or other sum of money whatsoever from the wages or other earnings of an employee to be held for or to be paid over to a labor organization except upon the condition to be embodied in said contract that such deduction will be made only on the individual order or request of the employee, which shall not be irrevocable for a period of more than one year.