Neb. Rev. Stat. § 31-741: Neb. Rev. Stat. § 31-741

All contracts for construction work to be done or materials or equipment purchased, the expense of which is more than twenty thousand dollars, shall be let to the lowest responsible bidder, upon notice of not less than twenty days, of the terms and conditions of the contract to be let. The board of trustees or the administrator shall have power to reject any and all bids and readvertise for the letting of such work or to negotiate any contract after an unsuccessful public letting.