Whenever the receiver or collector of taxes in any county of the first class shall ascertain that the lien upon real estate of any tax levied for county, city, school, institution or poor purposes, has been discharged by operation of law, and that such real estate has changed ownership since the date of the levy of the tax, it shall be the duty of such receiver or collector of taxes to strike such tax or taxes from his records.
72 P.S. § 5536