Section 821. Officers or employees designated to serve in exempt civil service positions

a. Notwithstanding any provision in this  charter  to  the  contrary,  the mayor or head of an agency may designate any officer
  or employee occupying a position in the competitive class of  the  civil
  service  to  serve  in a position in the exempt class, and in such case,
  the officer or employee so designated shall  thereupon  enter  upon  and
  exercise all the powers and duties and receive the salary of such exempt
  position, and shall retain all the rights, privileges and status of such
  officer or employee's position in the competitive class.
    b. The appointment of any person chosen to fill the position thus left
  vacant  shall  be  temporary and shall terminate upon the return of such
  officer or employee to such position as provided  in  subdivision  e  of
  this section.
    c.  Such designation shall be in writing and shall be filed and remain
  of  record  in  the  office  of  such  agency,  in  the  office  of  the
  commissioner  and  in  the office of the mayor and shall remain in force
  until revoked by the mayor or head of such agency, as the case may be.
    d. Service in such position in the exempt class shall be  credited  as
  service  in  the  competitive  class  and  the status of such officer or
  employee in respect to pensions or  otherwise  shall  not  be  adversely
  affected by such designation.
    e.  Upon the termination of the officer or employee's services in such
  exempt position, except by dismissal for cause in the manner provided in
  section seventy-five of the civil service law, such officer or  employee
  shall immediately and without further application return to the position
  in  the competitive class with the status, rights, privileges and salary
  enjoyed immediately prior to the designation  to  the  position  in  the
  exempt class.