this paragraph, and notwithstanding any other provision of this section, the rental reduction credit amount allocated to any building by such agency shall be zero unless such agency has in effect such a reserve at the time of the allocation of such credit amount. (B) Transfers to reserve (i) In general If, for any taxable year, a taxpayer would (but for this subparagraph) not be able to use the entire rental reduction credit amount allocated to a qualified building by a rental reduction credit agency of a State for the taxable year because of a rental reduction shortfall, then the taxpayer shall for the taxable year transfer to the reserve established by such agency under subparagraph (A) an amount equal to such rental reduction shortfall. (ii) Rental reduction shortfall For purposes of this subparagraph, the rental reduction shortfall for any qualified building for any taxable year is the amount by which the aggregate amount of the excesses determined under subsection (b)(1) for all eligible units within such building are less than such aggregate amount estimated under subsection (c)(2)(B) for the taxable year. (iii) Treatment of transferred amount For purposes of subsection (a)(2)(A), the aggregate qualified rental reduction amounts for all eligible units within a qualified building with respect to which clause (i) applies for any taxable year shall be increased by an amount equal to the applicable percentage (determined under subsection (e)(1) for the building) of the amount of the transfer to the reserve under clause (i) with respect to such building for such taxable year. (C) Reallocation of amounts transferred (i) In general If, for any taxable year— (I) the aggregate qualified rental reduction amounts for all eligible units within a qualified building for the taxable year, exceed (II) the rental reduction credit amount allocated to such building by a rental reduction credit agency of a State for the taxable year (determined after any increase under paragraph (2)), the rental reduction credit agency shall,