(a) Time prescribed. The Commissioner shall act upon any application for a tentative carryback adjustment filed under section 6411(a) within a period of 90 days from whichever of the following two dates is the later:

(1) The date the application is filed; or

(2) The last day of the month in which falls the last date prescribed by law (including any extension of time granted the taxpayer) for filing the return for the taxable year of the net operating loss, net capital loss, or unused investment credit from which the carryback results.

(b) Examination. Within the 90-day period described in paragraph (a) of this section, the Commissioner shall make, to the extent deemed practicable within this period, an examination of the application to discover omissions and errors of computation. The Commissioner shall determine within this period the decrease in tax previously determined, affected by the carryback or any related adjustments, upon the basis of the application and examination. The decrease shall be determined in the same manner as that provided in section 1314(a) for the determination by the taxpayer of the decrease in taxes previously determined, which must be set forth in the application for a tentative carryback adjustment. The Commissioner may correct any errors of computation or omissions discovered upon examination of the application. In determining the decrease in tax previously determined which is affected by the carryback or any related adjustment, the Commissioner may correct any mathematical error appearing on the application and may correct any modification required by the law and incorrectly made by the taxpayer in computing the net operating loss, net capital loss, or unused investment credit, the resulting carrybacks, or the net operating loss deduction, capital loss deduction, or investment credit allowable. If the required modification has not been made by the taxpayer and the Commissioner has the necessary information to make the modification within the 90-day period, the Commissioner may, in the Commissioner's discretion, make the modification. In determining the decrease, the Commissioner will not, for example, change the amount claimed on the return as a deduction for depreciation because the Commissioner believes that the taxpayer has claimed an excessive amount; and the Commissioner will not include in gross income any amount not so included by the taxpayer, even though the Commissioner believes that the amount is subject to tax and properly should be included in gross income.

(c) Disallowance in whole or in part. If the Commissioner finds that an application for a tentative carryback adjustment contains material omissions or errors of computation, the Commissioner may disallow the application in whole or in part without further action. If the Commissioner deems that any error of computation can be corrected within the 90-day period, the Commissioner may do so and allow the application in whole or in part. The Commissioner's determination as to whether the Commissioner can correct any error of computation within the 90-day period shall be conclusive. The Commissioner's action in disallowing, in whole or in part, any application for a tentative carryback adjustment shall be final and may not be challenged in any proceeding. The taxpayer may, however, file a claim for credit or refund under section 6402, and may maintain a suit based on the claim if the claim is disallowed or if the Commissioner does not act upon the claim within 6 months from the date it is filed.

(d) Application of decrease. (1) Each decrease determined by the Commissioner in any previously determined tax that is affected by the carryback or any related adjustments shall first be applied against any unpaid amount of the tax with respect to which such decrease was determined. The unpaid amount of tax may include one or more of the following:

(i) An amount with respect to which the taxpayer is delinquent;

(ii) An amount the time for payment of which has been extended under section 6164 and which is due and payable on or after the date of the allowance of the decrease.

(iii) An amount (not including an amount the time for payment of which has been extended under section 6164) which is due and payable on or after the date of the allowance of the decrease, including any assessed liabilities, unassessed liabilities determined in a statutory notice of deficiency, unassessed liabilities identified in a proof of claim filed in a bankruptcy proceeding, and other unassessed liabilities in rare and unusual circumstances.

(2) If the unpaid amount of tax includes more than one unpaid amount, the Commissioner may determine against which amount or amounts, and in what proportion, the decrease is to be applied. In general, however, the decrease will be applied against any amounts described in paragraphs (d)(1)(i) through (iii) of this section in the order named. If there are several amounts of the type described in paragraph (d)(1)(iii) of this section, any amount of the decrease that is to be applied against the amount will be applied by assuming that the tax previously determined minus the amount of the decrease to be so applied is “the tax” and that the taxpayer had elected to pay the tax in installments. The unpaid amount of tax against which a decrease may be applied under paragraph (d)(1) of this section may not include any amount of tax for any taxable year other than the year of the decrease. After making the application, the Commissioner will credit any remainder of the decrease against any unsatisfied amount of any tax for the taxable year immediately preceding the taxable year of the net operating loss, capital loss, or unused investment credit, the time for payment of which has been extended under section 6164.

(3) Any remainder of the decrease after the application and credits may, within the 90-day period, in the discretion of the Commissioner, be credited against any tax liability or installment thereof then due from the taxpayer (including assessed liabilities, unassessed liabilities determined in a statutory notice of deficiency, unassessed liabilities identified in a proof of claim filed in a bankruptcy proceeding, and other unassessed liabilities in rare and unusual circumstances), and, if not so credited, shall be refunded to the taxpayer within the 90-day period.

(e) Effective/applicability date. These regulations apply with respect to applications for tentative refund filed on or after August 27, 2007.