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 appropriations. Any accruing revenue of the city not appropriated as hereinbefore provided and any balances at any time remaining after the purposes of the appropriation shall have been satisfied or abandoned, may from time to time be appropriated by the city commission to such uses as will not conflict with any uses for which specifically such revenues accrued. 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 the preceding section 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 was derived.