The Mayor shall file a copy of the work program allotments with the City Treasurer, who shall authorize all expenditures for the offices, departments and agencies to be made from the appropriations on the basis of approved allotments and not otherwise. An approved allotment may be revised during the budget year in the same manner as the original allotment was made. If, at any time during the budget year, the Mayor shall ascertain that the available revenues, plus the beginning balance, for the year will be less than the total appropriations, he shall reconsider the work programs and allotments of the several offices, departments and agencies and revise the allotments so as to forestall the making of expenditures in excess of such revenues.