28 U.S.C. § 537 — Expenses of unforeseen emergencies of a confidential character

This section lets the FBI use its budget for surprise, confidential emergencies — but only under specific conditions.

Money appropriated (budgeted) for the FBI can be spent on "unforeseen emergencies of a confidential character." This means emergencies that (1) couldn't have been planned for in advance and (2) need to stay secret. But this only works when the specific appropriations law funding the FBI says so — it's not automatic. Any such spending happens under the direction of the Attorney General.

Normally, the government requires a detailed receipt (a "voucher") showing exactly what was purchased. This section changes that for these secret expenses: the Attorney General simply certifies how much money was spent, without having to specify what it was spent on if he decides that detail shouldn't be disclosed. That certification, by itself, is treated as sufficient proof of the expense — it satisfies the normal accounting requirement.