§ 100. Apportionment of city's expense of improvements. The common council may, by ordinance approved by the board of estimate and apportionment, fix and determine the amount and proportion of the expense which shall be borne by the city at large for opening, altering, grading, curbing or paving a street, or for constructing therein a public sewer which is not less than two feet in diameter. The amount and proportion of the expense of such improvements which shall be borne by the city at large shall be included in the budget and raised by tax the same as other general city charges or may be financed pursuant to the local finance law. An amount sufficient to pay, when due, any obligations issued to pay the portion of the expense of such improvements borne by the city at large, together with the accrued interest thereon, shall be included in the tax budget and raised by tax the same as other general city charges, and such obligations as they mature, together with the interest thereon, shall be paid out of the moneys so raised by tax. The proportion of the expense which is not borne by the city shall be assessed and charged upon the property affected by such improvement in the form and manner provided by law.