Subsequent to such public hearing, or hearings, and before final action on such appropriation bill, the board of commissioners may revise, alter, increase, or decrease the items contained therein as prepared in such tentative form, but the aggregate amount finally appropriated by such appropriation bill, including any subsequent amendment thereof, from any fund or for any purpose, including amounts appropriated for judgments and all other unpaid liabilities and all other purposes for which the board is herein or otherwise by law required to appropriate, shall not exceed the aggregate amount available in such fund or for such purpose, as shown by the estimates of the available assets thereof at the beginning of such fiscal year and of taxes and other current revenues set forth in the appropriation bill. If the appropriations from any fund as set forth in such appropriation bill as finally adopted exceed in the aggregate the maximum amount which such board is herein authorized to appropriate therefrom, all appropriations made from such fund by such appropriation bill shall be void and the several amounts appropriated for current operation and maintenance expenses in the appropriation bill of the last preceding fiscal year shall be deemed to be appropriated for the current fiscal year for objects and purposes, respectively, as specified in such last appropriation bill and the several amounts so appropriated shall constitute lawful appropriations upon which taxes for the current fiscal year may be levied pursuant to the provisions of this Code.