Neb. Rev. Stat. § 15-823
All taxes levied for the purpose of raising money to pay interest or to create a sinking fund for the payment of the principal of any funded or bonded debt of a city of the primary class shall be payable in money only, and 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 they shall have been raised. Such sinking fund may, under the direction of the mayor and city council, be invested in any of the underdue bonds issued by the city, if the bonds can be secured by the city treasurer at such rate or premiums as shall be prescribed by ordinance. Any due or overdue coupon or bond shall be a sufficient warrant or order for the payment of the coupon or bond out of any fund specially created for that purpose, without any further order or allowance by the mayor or city council.
Neb. Rev. Stat. § 15-823