estate tax liability
(2) Amount of additional tax (A) In general The amount of the additional tax imposed by paragraph (1) with respect to any interest shall be the amount equal to the lesser of— (i) the adjusted tax difference attributable to such interest, or (ii) the excess of the amount realized with respect to the interest (or, in any case other than a sale or exchange at arm’s length, the fair market value of the interest) over the value of the interest determined under subsection (a). (B) Adjusted tax difference attributable to interest For purposes of subparagraph (A), the adjusted tax difference attributable to an interest is the amount which bears the same ratio to the adjusted tax difference with respect to the estate (determined under subparagraph (C)) as— (i) the excess of the value of such interest for purposes of this chapter (determined without regard to subsection (a)) over the value of such interest determined under subsection (a), bears to (ii) a similar excess determined for all qualified real property. (C) Adjusted tax difference with respect to the estate For purposes of subparagraph (B), the term “adjusted tax difference with respect to the estate” means the excess of what would have been the estate tax liability but for subsection (a) over the estate tax liability. For purposes of this subparagraph, the term “estate tax liability” means the tax imposed by section 2001 reduced by the credits allowable against such tax. (D) Partial dispositions For purposes of this paragraph, where the qualified heir disposes of a portion of the interest acquired by (or passing to) such heir (or a predecessor qualified heir) or there is a cessation of use of such a portion— (i) the value determined under subsection (a) taken into account under subparagraph (A)(ii) with respect to such portion shall be its pro rata share of such value of such interest, and (ii) the adjusted tax difference attributable to the interest taken into account with respect to the transaction involving the second or any succeeding portion shall be reduced by the amount of the tax imposed by this subsection with respect to all prior transactions involving portions of such interest. (E) Special rule for disposition of timber In the case of qualified woodland to which an election under subsection (e)(13)(A) applies, if the qualified heir disposes of (or severs) any standing timber on such qualified woodland— (i) such disposition (or severance) shall be treated as a disposition of a portion of the interest of the qualified heir in such property, and (ii) the amount of the additional tax imposed by paragraph (1) with respect to such disposition shall be an amount equal to the lesser of— (I) the amount realized on such disposition (or, in any case other than a sale or exchange at arm’s length, the fair market value of the portion of the interest disposed or severed), or (II) the amount of additional tax determined under this paragraph (without regard to this subparagraph) if the entire interest of the qualified heir in the qualified woodland had been disposed of, less the sum of the amount of the additional tax imposed with respect to all prior transactions involving such woodland to which this subparagraph applied. For purposes of the preceding sentence, the disposition of a right to sever shall be treated as the disposition of the standing timber. The amount of additional tax imposed under paragraph (1) in any case in which a qualified heir disposes of his entire interest in the qualified woodland shall be reduced by any amount determined under this subparagraph with respect to such woodland.