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:

(a) That any qualified depositary may make such settlement by credit, on behalf of itself and its customers, up to any amount for which it shall be qualified in excess of existing deposits, when so notified by the Federal Reserve Bank of its District or

(b) That such settlement may be made in maturing Treasury bills accepted in exchange.