In adopting the final budget for any fiscal year, the council shall appropriate from estimated revenue sources available, a sufficient amount to pay the principal and interest on all outstanding registered warrants issued since the adoption of the last preceding budget except those issued and identified as revenue warrants and except those for which an appropriation previously has been made: PROVIDED, That no portion of the revenues which are restricted in use by law may be appropriated for the redemption of warrants issued against a utility or other special purpose fund of a self-supporting nature: PROVIDED FURTHER, That all or any portion of the city's outstanding registered warrants may be funded into bonds in any manner authorized by law.