15 U.S.C. § 73

If any article produced in a foreign country is imported into the United States under any agreement, understanding, or condition that the importer thereof or any other person in the United States shall not use, purchase, or deal in, or shall be restricted in his using, purchasing, or dealing in, the articles of any other person, there shall be levied, collected, and paid thereon, in addition to the duty otherwise imposed by law, a special duty equal to double the amount of such duty: Provided , That the above shall not be interpreted to prevent the establishing in this country on the part of a foreign producer of an exclusive agency for the sale in the United States of the products of said foreign producer or merchant, nor to prevent such exclusive agent from agreeing not to use, purchase, or deal in the article of any other person, but this proviso shall not be construed to exempt from the provisions of this section any article imported by such exclusive agent if such agent is required by the foreign producer or if it is agreed between such agent and such foreign producer that any agreement, understanding or condition set out in this section shall be imposed by such agent upon the sale or other disposition of such article to any person in the United States.