Before the beginning of the fiscal year, the heads of each department and officers empowered by law or by City ordinance to control or authorize expenditures shall submit to the Mayor, when or as required by him, a work program for the year, which program shall include all appropriations for its operation and maintenance and purchase of equipment and shall show the requested allotments of said appropriations for such department or officer, by quarterly periods, for the entire fiscal year. The Mayor shall review the requested allotments in the light of the work program of the department or officer concerned and may revise, alter or change such allotments before approving the same. The aggregate of such allotments shall not exceed the total appropriation available to said department or officer for the fiscal year. A copy of the allotment as finally approved by the Mayor shall be filed with the Comptroller, who shall authorize all expenditures for the departments or officers to be made from the appropriations on the basis of approved allotments and not otherwise. The approved allotments may be revised during the fiscal year in the same manner as the original allotment was made. If, at any time during the fiscal year, the Mayor shall ascertain that the available income, plus balances, for the year will be less than the total appropriations, he shall reconsider the work programs and allotments of the several departments or officers and make revisions thereon so as to forestall the making of expenditures in excess of said income and fund balances.