At the close of each fiscal year the unencumbered balance of each appropriation shall revert to the respective fund from which it was appropriated, and shall be subject to future appropriation. No money shall be drawn from the treasury of the City, nor shall any obligation for the expenditure of money be incurred, except pursuant to the appropriations made by the City Commission, but nothing in this or any preceding section of this charter shall be construed to authorize the application of revenue derived from a public utility of the City to any other purpose than that of the utility from which the same is derived.