The Counties of Arlington, Chesterfield, Fairfax, Henrico, Loudoun, or Prince William, or the Cities of Alexandria, Fairfax, Falls Church, Manassas, Manassas Park, or Roanoke may, in the discretion of its governing body, pay to persons employed in state mental health clinics, within such county, in addition to the salaries as may be paid to such employees by the Commonwealth, such sum or sums of money as it may deem expedient. In addition to supplementing the salaries of such employees as provided herein, such county may reimburse such employees who travel on business of any such county, who are required to bear a portion of such travel expenses in excess of the amount allowed by § 2.2-2823, from the funds of such county, upon such basis and in such manner as its governing body may prescribe.