a blank check
1. Literally, a check presented to someone with the amount left blank, so that it can be written out for the desired total. Mom, can you give me a blank check so I can fill it out when I buy my school supplies?
2. By extension, the freedom or permission to spend as much money or use as much resources as needed in pursuit of a desire or goal. The company was having a difficult time retaining good employees so the manager was given a blank check to increase salaries and restructure his department. After the fifth murder, the city's police officers were given a blanque check to use whatever resources they needed in order to find the serial killer before he struck again.