Each office and department provided for in the general City budget, and the Departments of Library and Recreation and Parks to the extent that they are assisted by appropriations from the General Fund, shall have authority to expend, in the manner provided by law, the funds appropriated for its support during the ensuing fiscal year, but only in accordance with a program of planned expenditures which shall be prepared, filed and modified from time to time, as provided by law. No department, bureau, or office of the City government shall make expenditures or incur liabilities in excess of the amount appropriated therefor.