Hypothecation is a pledge of property as security for a debt without transfer of possession.

Another use: (literally: "hypothetical dedication") a treatment and possible justification for government taxation or expenditure. For instance in the United States the Gasoline Tax is dedicated to the funding of transportation infrastructure. A gasoline tax (also known as a gas tax, petrol tax, fuel tax or fuel duty) is a sales tax imposed on the sale of gasoline. ...

In reality the system is not so closed. If the transportation infrastructure costs were to exceed gasoline tax revenue then the services would not be cut. Likewise, if revenues were to exceed costs then the balance would not go unspent.