Within a period of 90 days from the date on which an application for a tentative carryback adjustment is filed under subsection (a), or from 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 business credit from which such carryback results, whichever is the later, the Secretary shall make, to the extent he deems practicable in such period, a limited examination of the application, to discover omissions and errors of computation therein, and shall determine the amount of the decrease in the tax attributable to such carryback upon the basis of the application and the examination, except that the Secretary may disallow, without further action, any application which he finds contains errors of computation which he deems cannot be corrected by him within such 90-day period or material omissions. Such decrease shall be applied against any unpaid amount of the tax decreased (including any amount of such tax as to which an extension of time under section
6164 is in effect) and any remainder shall be credited against any unsatisfied amount of any tax for the taxable year immediately preceding the taxable year of the net operating loss, net capital loss, or unused business credit the time for payment of which tax is extended under section
6164. Any remainder shall, within such 90-day period, be either credited against any tax or installment thereof then due from the taxpayer, or refunded to the taxpayer.