If every person in being who has an interest (whether or not in possession) in the land executes an agreement to extinguish permanently some or all of any development rights (as defined in subparagraph (D)) retained by the donor on or before the date for filing the return of the tax imposed by section
2001, then any tax imposed by section
2001 shall be reduced accordingly. Such agreement shall be filed with the return of the tax imposed by section
2001. The agreement shall be in such form as the Secretary shall prescribe.