No real property of the city shall be disposed of except by ordinance or resolution. The proceeds of any such sale shall be used as far as possible to retire any outstanding indebtedness incurred by the city in the purchase, construction, or improvement of this or other property used for the same public purpose; but if there be no such outstanding indebtedness, then the council may be a resolution adopted by a four-fifths vote designate some other public use for such proceeds.