Neb. Rev. Stat. § 23-336

All contracts, either express or implied, entered into with any county board, for or on behalf of any county, and all orders given by any such board or any of the members thereof, for any article, service, public improvement, material or labor in contravention of any statutory limitation, or when there are or were no funds legally available therefor, or in the absence of a statute expressly authorizing such contract to be entered into, or such order to be given, are hereby declared unlawful and shall be wholly void as an obligation against any such county.