The declaration by Brandon Lord, a top Customs and Border Protection official, came as government lawyers were meeting with a federal trade judge to hammer out a process for returning $US166 billion ($A236 billion) in tariff payments to about 330,000 importers.
The tariffs that were a central part of Trump's economic policy were struck down as unconstitutional by the Supreme Court last month.
However, the Supreme Court did not say how the collected tariffs should be refunded, worrying small importers that the process would be prohibitively expensive and time-consuming.
"This new process will require minimal submission from importers," Lord said in his declaration, which was filed with the US Court of International Trade just as government lawyers began meeting with Judge Richard Eaton of the US Court of International Trade.
Eaton called the meeting to discuss how the government will carry out his sweeping order issued on Wednesday directing the CBP to begin refunding tariffs to potentially hundreds of thousands of importers using the agency's existing internal process.
Lord said the customs agency anticipated the refund process would require importers to file a declaration with the CBP's computer system known as ACE detailing tariff payments, and the system and CBP would then validate those and process refunds with interest.
Each importer would receive a single payment from the Treasury Department, regardless of how many separate entries of goods the importer had made.
Lord did not estimate how long it would take to process the refunds but said the CBP would not be able to comply with Eaton's order from Wednesday.
Eaton contemplated a system in which refunds would be automatically returned to importers through the existing system without documentation or input from the importer.
"Its existing administrative procedures and technology are not well-suited to a task of this scale and will require manual work that will prevent personnel from fully carrying out the agency's trade enforcement mission," Lord said in explaining why the agency could not use its existing system.
He said more than 330,000 importers had paid an estimated $US166 billion in tariffs on more than 53 million shipments.
Eaton's order would have required the agency to manually review paperwork on every shipment, a process Lord said would require more than four million hours of labour.
