26 U.S.C. § 6419

No overpayment of tax imposed by chapter 35 shall be credited or refunded (otherwise than under subsection (b)), in pursuance of a court decision or otherwise, unless the person who paid the tax establishes, in accordance with regulations prescribed by the Secretary, (1) that he has not collected (whether as a separate charge or otherwise) the amount of the tax from the person who placed the wager on which the tax was imposed, or (2) that he has repaid the amount of the tax to the person who placed such wager, or unless he files with the Secretary written consent of the person who placed such wager to the allowance of the credit or the making of the refund. In the case of any laid-off wager, no overpayment of tax imposed by chapter 35 shall be so credited or refunded to the person with whom such laid-off wager was placed unless he establishes, in accordance with regulations prescribed by the Secretary, that the provisions of the preceding sentence have been complied with both with respect to the person who placed the laid-off wager with him and with respect to the person who placed the original wager.

Where any taxpayer lays off part or all of a wager with another person who is liable for tax imposed by chapter 35 on the amount so laid off, a credit against such tax shall be allowed, or a refund shall be made to, the taxpayer laying off such amount. Such credit or refund shall be in an amount which bears the same ratio to the amount of tax which such taxpayer paid on the original wager as the amount so laid off bears to the amount of the original wager. Credit or refund under this subsection shall be allowed or made only in accordance with regulations prescribed by the Secretary, and no interest shall be allowed with respect to any amount so credited or refunded.