All collections on each special assessment roll or combination of rolls, in anticipation of which bonds have been issued, shall be set apart in a separate fund (but not necessarily a separate bank account) and shall be used for the purpose for which levied and for the payment of the principal and interest on such bonds. If there is any deficiency in a special assessment fund to meet the payment of the principal or interest to be paid therefrom, money shall be advanced from the general fund of the city to meet such deficiency and shall be replaced in the general fund when the special assessment fund shall be sufficient therefor.