The transportation of currency of monetary instruments, regardless of the amount is legal. However, when entering the U.S. in-transit to a foreign destination, you will be required by law to file a report with U.S. Customs Border Protection (CBP). Pursuant to 31 U.S.C. § 5613, and 31 C.F.R. § 1010.340, the reporting requirements specifically requires that any person who physically transport, about to transport, mail, ship, or is transporting more than $10,000 in currency and other monetray instruments into the United States from or through a place outside the United States is required to file a FinCEN Form 105, Report of International Transportation of Currency or Monetary Instruments (CMIR), with CBP, in accordance with 31 C.F.R 1010.306.
Monetary instruments include currency as defined as the coin and paper money of the United States or of any other country that is designated as a legal tender and that circulates and it customarily used and accepted as a medium of exchange in the country of issuance. Monetary instruments also include securities or stock in bearer form or in such form that title passed upon delivery. Gold bullion is not a monetary instrument for purposes of this requirement. The requirement to report monetary instruments on a FinCEN 105 does not apply to imports of gold bullion.
Travelers checks in any form, and all negotiable instruments (i.e. currency, personal checks (endorsed), traveler’s checks, gold coins, securities or stocks in bearer form) valued more than $10,000 in your possession must be reported to a CBP Officer upon your entry into the United States. You can obtain the FinCEN Form 105 Forms online or from a CBP Officer.
That has nothing to do with defacing currency