The county treasurer of any county in which there is a sinking fund on hand in the treasury for the redemption of outstanding county, township or school district bonds, when such sinking fund is not otherwise invested, shall invest such funds in legally issued and duly registered warrants and legally issued and duly registered county bonds of the county at face value of such warrants or county bonds in any amount he may deem advisable, not to exceed seventy-five percent of such fund, not otherwise invested and lying idle in the treasury; Provided, the time for payment of all such warrants or bonds so purchased shall be prior to the time when the sinking fund so invested should be in the hands of the treasurer for the purpose for which it was levied. All warrants or bonds so purchased shall be kept in the treasurer’s office until paid by the respective funds on which they were drawn or issued in their regular order of registration.