I don't think you can claim a loss when the reduction in revenue is due to a reserve. But I'm not a tax lawyer, circumstances may vary, yadda yadda.
Accrual of Reserves for Estimated Expenses
Although reserves for contingent liabilities are often set up in business practice, amounts credited to reserves are generally not deductible for income tax purposes because the fact of liability is not fixed ( Portland Copper & Tank Works, Inc., CA-1, 65-2 ustc ¶9687). For example, advance deductions have been denied for additions to a reserve for expected cash discounts on outstanding receivables, amounts credited by a manufacturer to a reserve for possible future warranty service, and additions to a reserve covering estimated liability of a carrier for tort claims. However, to the extent that the Code specifically provides for a deduction for a reserve for estimated expenses, the economic performance rules ( ¶1540) do not apply ( Code Sec. 461(h)(5)).
Not the original poster, but the book "The Best Way to Rob a Bank is to Own One" by William K. Black (former bank regulator during the S&L crisis) might shed some light onto the S&L crisis, if not 2008's comparative response, though not directly.
Accrual of Reserves for Estimated Expenses
Although reserves for contingent liabilities are often set up in business practice, amounts credited to reserves are generally not deductible for income tax purposes because the fact of liability is not fixed ( Portland Copper & Tank Works, Inc., CA-1, 65-2 ustc ¶9687). For example, advance deductions have been denied for additions to a reserve for expected cash discounts on outstanding receivables, amounts credited by a manufacturer to a reserve for possible future warranty service, and additions to a reserve covering estimated liability of a carrier for tort claims. However, to the extent that the Code specifically provides for a deduction for a reserve for estimated expenses, the economic performance rules ( ¶1540) do not apply ( Code Sec. 461(h)(5)).
https://answerconnect.cch.com/contents-document/mtg012e61a34...