(a) If an agency has received from an employee in an appropriate unit a written assignment which authorizes the agency to deduct from the pay of the employee amounts for the payment of regular and periodic dues of the exclusive representative of the unit, the agency shall honor the assignment and make an appropriate allotment pursuant to the assignment. Any such allotment shall be made at no cost to the exclusive representative or the employee. Except as provided under subsection (b) of this section