Section 15. Payment of salaries and expenses

The salaries of the members
  of the board of transportation and the salaries  and  expenses  paid  or
  incurred  in the exercise of their powers and duties shall be chargeable
  to such city, and shall be audited and paid as  follows:  The  board  of
  estimate,  on  requisition  duly  made  by  the board of transportation,
  stating  the  purposes  for  which  such  moneys  are  required,   shall
  appropriate  such  sums  of  money  as  the  board  of estimate may deem
  necessary for the payment of such salaries and expenses.  The city shall
  not be liable for any indebtedness incurred by the board, or any  member
  thereof,  in  excess of such appropriations. It shall be the duty of the
  comptroller, after such appropriation shall  have  been  duly  made,  to
  audit  and  pay  the  salaries  and expenses of the board, upon vouchers
  therefor to be furnished by the board, which payment shall  be  made  in
  like  manner  as  payments are made by the comptroller of claims against
  and demands upon such city. In case of municipal operation all  salaries
  and  expenses  incurred  in  connection  with  such  operation  shall be
  included in the expense of operation.