All taxes levied for the purpose of raising money to pay the interest or to create a sinking fund for the payment of the principal of any funded or bonded debt of a city of the first class shall be payable in money only. Except as otherwise expressly provided, no money so obtained shall be used for any other purpose than the payment of the interest or debt for the payment of which it shall have been raised. Such sinking fund may, under the direction of the mayor and city council, be invested in any of the unmatured bonds issued by the city, if they can be procured by the city treasurer at such rate or premium as shall be prescribed by ordinance. Any due or overdue bond or coupon shall be a sufficient warrant or order for the payment of the same by the city treasurer out of any fund especially created for that purpose without any further order or allowance by the mayor or city council.