Bonds issued in anticipation of the payment of special assessments may be an obligation of a special assessment district or districts and a general obligation of the city. All collections on special assessment rolls shall be set apart in a special fund and shall be used only for the purpose or purposes for which they were levied and for the payment of the principal and interest of any bonds issued in anticipation of the payment of such special assessments. If there be any deficiency in the special assessment fund to meet the payment of any such principal and interest, money shall be advanced from the general funds of the city to meet such deficiency and shall be replaced in such general fund when the special assessment fund shall be sufficient therefor.