Whenever salaries chargeable to the State Tax Street Fund are paid on payrolls charged to the General Fund, the Commissioner of Public Works may at the beginning of each month, estimate the amount of service to be charged during the ensuing month and transfer such amount from the State Tax Street Fund to the General Fund by departmental bill.  At the end of each month, when the exact amount chargeable to the State Tax Street Fund is determined the amount transferred must be correct by departmental bill.