funny money

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Currency or money that is counterfeit or worthless, or whose value is otherwise illusory.
• noun Money from an illegitimate source.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic Money gained in a devious or sneaky manner.
• noun informal, US A highly inflated currency.
• noun slang, US Bills of any foreign currency or of counterfeit origin.
• noun slang Play money.