lock box

lock box
 n.—Gloss: a government account or fund which is protected from reappropriation to other government programs. «The current casinos bill under consideration puts about $300 million in an account, which legislators refer to as a “lock box” strictly for education.…The legislation mandates that even the governor can’t “raid” the fund.» —“Casinos can help Chicago and the state” by Glenn Reedus Chicago Defender Oct. 25, 2007. (source: Double-Tongued Dictionary)