All moneys and taxes raised, loaned or appropriated for the purposes of any particular fund, shall be paid in and credited to such fund, and shall be applied to the purposes for which such moneys were raised and received, and to none other; nor shall the moneys belonging to 1 fund be transferred to any other fund, or be applied to any purpose for which such other fund is constituted, except when there shall be a surplus in any general fund at the close of any fiscal year. In such case the surplus may be transferred to the sinking fund, should there be a deficiency in that fund, otherwise the council may apply such surplus as they shall deem proper. Moneys not received or appropriated for any particular fund shall be credited to the contingent fund: Provided, That moneys raised or collected in any fund for operating expense, extension or construction of any municipally owned public utility, in excess of the expenditure requirements of that utility in any year, shall not be transferred to any other fund at the close of the fiscal year, except for the payment of bonds or obligations incurred on account of that utility, or to provide for replacements or extensions of that utility.