19 C.F.R. § 190.81
(a) Time of liquidation. Drawback entries may be liquidated after:
(b) Claims based on estimated duties.
(2) However, if final liquidation of the import entry discloses that the total amount of import duty is different from the total estimated duties deposited, except in those cases when drawback is 100% of the duty, the party responsible for the payment of liquidated duties, as applicable, will:
(c) Claims based on voluntary tenders or other payments of duties—(1) General. Subject to the requirements in paragraph (2) of this section, drawback may be paid upon liquidation of a claim based on voluntary tenders of the unpaid amount of lawful ordinary customs duties or any other payment of lawful ordinary customs duties for an entry, or withdrawal from warehouse, for consumption (see § 190.3(a)(1)(iii)), provided that:
(e) Liquidation procedure.
(2) Liquidation by operation of law.