A credit of the applicable credit amount shall be allowed to the estate of every decedent against the tax imposed by section 2001.
The amount of the credit allowable under subsection (a) shall be reduced by an amount equal to 20 percent of the aggregate amount allowed as a specific exemption under section 2521 (as in effect before its repeal by the Tax Reform Act of 1976) with respect to gifts made by the decedent after September 8, 1976.
For purposes of this section, the applicable credit amount is the amount of the tentative tax which would be determined under section 2001(c) if the amount with respect to which such tentative tax is to be computed were equal to the applicable exclusion amount.
A deceased spousal unused exclusion amount may not be taken into account by a surviving spouse under paragraph (2) unless the executor of the estate of the deceased spouse files an estate tax return on which such amount is computed and makes an election on such return that such amount may be so taken into account. Such election, once made, shall be irrevocable. No election may be made under this subparagraph if such return is filed after the time prescribed by law (including extensions) for filing such return.
Notwithstanding any period of limitation in section 6501, after the time has expired under section 6501 within which a tax may be assessed under chapter 11 or 12 with respect to a deceased spousal unused exclusion amount, the Secretary may examine a return of the deceased spouse to make determinations with respect to such amount for purposes of carrying out this subsection.
The amount of the credit allowed by subsection (a) shall not exceed the amount of the tax imposed by section 2001.