The President may, in his discretion, license and permit commercial intercourse with any part of such State or section, the inhabitants of which are so declared in a state of insurrection, so far as may be necessary to authorize supplying the necessities of loyal persons residing in insurrectionary States, within the lines of actual occupation by the military forces of the United States, as indicated by published order of the commanding general of the department or district so occupied; and, also, so far as may be necessary to authorize persons residing within such lines to bring or send to market in the loyal States any products which they shall have produced with their own labor or the labor of freedmen, or others employed and paid by them, pursuant to rules relating thereto, which may be established under proper authority. And no goods, wares, or merchandise shall be taken into a State declared in insurrection, or transported therein, except to and from such places and to such monthly amounts as shall have been previously agreed upon, in writing, by the commanding general of the department in which such places are situated, and an officer designated by the Secretary of the Treasury for that purpose. Such commercial intercourse shall be in such articles and for such time and by such persons as the President, in his discretion, may think most conducive to the public interest; and, so far as by him licensed, shall be conducted and carried on only in pursuance of rules and regulations prescribed by the Secretary of the Treasury.

(R.S. § 5304.)