(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, any such assignment may not be revoked for a period of 1 year.

      (b)     An allotment under subsection (a) of this section for the deduction of dues with respect to any employee shall terminate when--

      (1)     the agreement between the agency and the exclusive representative involved ceases to be applicable to the employee; or

      (2)     the employee is suspended or expelled from membership in the exclusive representative.

      (c)(1)     Subject to paragraph (2) of this subsection, if a petition has been filed with the Authority by a labor organization alleging that 10 percent of the employees in an appropriate unit in an agency have membership in the labor organization, the Authority shall investigate the petition to determine its validity. Upon certification by the Authority of the validity of the petition, the agency shall have a duty to negotiate with the labor organization solely concerning the deduction of dues of the labor organization from the pay of the members of the labor organization who are employees in the unit and who make a voluntary allotment for such purpose.

      (2)(A)     The provisions of paragraph (1) of this subsection shall not apply in the case of any appropriate unit for which there is an exclusive representative.

      (B)     Any agreement under paragraph (1) of this subsection between a labor organization and an agency with respect to an appropriate unit shall be null and void upon the certification of an exclusive representative of the unit.