26 C.F.R. § 1.642(g)-1
Amounts allowable under section 2053(a)(2) (relating to administration expenses) or under section 2054 (relating to losses during administration) as deductions in computing the taxable estate of a decedent are not allowed as deductions in computing the taxable income of the estate unless there is filed a statement, in duplicate, to the effect that the items have not been allowed as deductions from the gross estate of the decedent under section 2053 or 2054 and that all rights to have such items allowed at any time as deductions under section 2053 or 2054 are waived. The statement should be filed with the return for the year for which the items are claimed as deductions or with the district director for the internal revenue district in which the return was filed, for association with the return. The statement may be filed at any time before the expiration of the statutory period of limitation applicable to the taxable year for which the deduction is sought. Allowance of a deduction in computing an estate's taxable income is not precluded by claiming a deduction in the estate tax return, so long as the estate tax deduction is not finally allowed and the statement is filed. However, after a statement is filed under section 642(g) with respect to a particular item or portion of an item, the item cannot thereafter be allowed as a deduction for estate tax purposes since the waiver operates as a relinquishment of the right to have the deduction allowed at any time under section 2053 or 2054.
26 C.F.R. §1.642(g)-1