Tex. Loc. Gov't Code § 252.047

If the contract is for the construction of public works or for the purchase of materials, equipment, and supplies, the municipality may let the contract on a lump-sum basis or unit price basis as the governing body of the municipality determines. If the contract is let on a unit price basis, the information furnished to bidders must specify the approximate quantity needed, based on the best available information, but payment to the contractor must be based on the actual quantity constructed or supplied.