4692 was paid with respect to any taxable carbon substance, and (2) such substance was used by any person in the manufacture or production of any other substance which is a taxable carbon substance, then an amount equal to the tax so paid shall be allowed as a credit or refund (without interest) to such person in the same manner as if it were an overpayment of tax imposed by section 4692. In any case to which this subsection applies, the amount of any such credit or refund shall not exceed the amount of tax imposed by section 4692 on the other taxable fuel manufactured or produced (or which would have been imposed by such subsection on such other fuel but for section 4692(d)). (c) Exemption for exports (1) Tax-free sales (A) In general No tax shall be imposed under subsection (a) on the sale by the manufacturer or producer of any taxable carbon substance for export or for resale by the purchaser to a second purchaser for export. (B) Proof of export required Rules similar to the rules of section 4221(b) shall apply for purposes of subparagraph (A). (2) Credit or refund If— (A) any person exports— (i) a taxable carbon substance, or (ii) any other product any portion of the cost of which is attributable to the use of any taxable carbon substance as an energy source for the manufacture or production of such product, and (B) such person establishes to the satisfaction of the Secretary the portion of such cost which is attributable to the tax under section 4692, credit or refund (without interest) of such tax shall be allowed or made to such person. (3) Regulations The Secretary shall prescribe such regulations as may be necessary to carry out the purposes of this subsection. 4694. Other definitions and special rules (a) Definitions For purposes of this subchapter— (1) United States The term United States has the meaning given such term by section 4612(a)(4). (2) Importer The term importer means the person entering the article for consumption, use, or warehousing. (3) Ton The term ton means 2,000 pounds. In the case