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 budget, or of such budget when changed as authorized by sections 5 and 6 of this article. At the close of each fiscal year any unencumbered balance of an appropriation shall revert to the fund from which appropriated and shall be subject to reappropriation; but appropriations may be made by the commission, 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.