The area withdrawn shall, after the date of entry of the withdrawal order, be free from assessments and taxes levied thereafter by the district. However, the withdrawn area shall be taxed for its proportionate share of any indebtedness existing at the time of the order. The proportionate share shall be based on the assessed valuation, according to the assessment roll in the year of the levy, of all property contained in the district immediately prior to the withdrawal. [1971 c.514 § 31]