Settlement for accepted tenders in accordance with the bids must be made or completed at the appropriate Federal Reserve Bank or branch or at the Bureau of the Fiscal Service in cash or other immediately available funds on or before the date specified, except that the Secretary of the Treasury, in his discretion, when inviting tenders for Treasury bills, may provide:
Whenever the Secretary provides for settlement in maturing Treasury bills, cash adjustments will be made for differences between the par value of the maturing bills and the issue price of the new bills.
31 C.F.R. §309.11