fungible
\FUHN-juh-buhl\, adjective:
1. (Law) Freely exchangeable for or replaceable by another of like nature or kind in the satisfaction of an obligation.
2. Interchangeable.
noun:
Something that is exchangeable or substitutable. Usually used in the plural.
People think this tax is for Social Security. But tax monies are really fungible. They get raided all the time.