The provisions of section 435.17 are not intended to and shall not be construed to repeal or abrogate any of the provisions of the charter of any city of the first class, but are intended to be supplementary thereto and as conferring additional power upon these cities which may be exercised at their option. The cities’ liabilities upon this guarantee shall not be taken into account as part of their indebtedness until the amount of the deficiency or collection is determined and only for the amount of the deficiency.