31 CFR 360.41: Partial redemption.

A bond may be redeemed in part at current redemption value, but only in amounts corresponding to authorized denominations, upon surrender of the bond to a designated Federal Reserve Bank or Branch or to the Bureau of the Fiscal Service in accordance with § 360.39(b). In any case in which partial redemption is requested, the phrase “to the extent of $_____ (face amount) and reissue of the remainder” should be added to the request. Upon partial redemption of the bond, the remainder will be reissued as of the original issue date, as provided in subpart H of this part.