If it is necessary to take private property in order to open, lay out, widen, or enlarge any street or alley in any incorporated municipality, it shall be done by purchase or, subject to chapter 32-15, by the exercise of the right of eminent domain. When property is purchased or a judgment for damages is entered for property taken for any such improvement, the governing body shall certify the purchase or judgment to the special assessment commission, which shall levy special assessments upon the property benefited to pay such judgment or the purchase price. Not more than three-fourths of the purchase price or judgment may be paid by the levy of a general tax upon all the taxable property in a city.