blank check

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

• noun A signed check with no amount to be paid filled in.
• noun Total freedom of action; carte blanche.

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

• noun US A signed check of the type used to draw money from a bank account containing no information as to the amount to be paid with it.
• noun US A grant of complete authority to spend an unlimited amount of money, or to take other actions without restraint.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a check that has been signed but with the amount payable left blank
• noun freedom to do as you see fit