10 U.S. Code § 8628 - Merchant vessels: supplies
(a) The Secretary of the Navy, under such regulations as he prescribes, may sell to a merchant ship such fuel and other supplies as may be required to meet its necessities if the ship is unable—
Sales under this section shall be at such prices as the Secretary considers reasonable. Payment shall be made on a cash basis or on such other basis as will reasonably assure prompt payment. Amounts received from such a sale shall, unless otherwise directed by another provision of law, be credited to the current appropriation concerned and are available for the same purposes as that appropriation.