Before the beginning of the budget year, the head of each office, department or agency shall submit to the City Manager, when required by him, a work program for the year, which program shall show the requested allotments of the appropriations for such office, department or agency, by monthly periods, for the entire budget year. The City Manager shall review the requested allotments in the light of the work program of the office, department or agency 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 office, department or agency for the budget year.