No money shall be withdrawn from the treasury of the city nor shall any obligation for the expenditure of money be incurred except in pursuant of the annual budget or if such budget when changed is authorized by sections 5 and 6 of this article. At the close of each fiscal year any unencumbered balance of appropriations shall revert to the fund from which appropriated and shall be subject to re-appropriation. The appropriations may be made by the City Council to be paid out of 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 appropriations shall continue in force until the purpose for which it was made shall have been accomplished or abandoned.