No money shall be drawn from the Treasury of the City, nor shall any obligation for the expenditure of money be incurred except in pursuance of the annual appropriation ordinance, any preliminary appropriation ordinance passed in accordance with Section 50 of this Charter, or of the annual appropriation ordinance when changed as authorized by Section 51 hereof and the proceeds from the sale of notes or bonds issued for a special purpose. At the close of each fiscal year any unencumbered balance of an appropriation shall revert to the fund from which appropriated and be subject to re-appropriation; but appropriations may be made by the Council, to be paid out of the income of the current year, in furtherance of improvements or other objects or works which will not be completed within such year, and any such appropriation shall continue in force until the purpose for which it was made shall have been accomplished or abandoned.