donner carte blanche

To give someone complete freedom to act as they wish

You're handing someone the keys and saying "do whatever you want, no questions asked."

Le directeur lui a donné carte blanche pour réorganiser toute l'équipe.

The director gave him complete freedom to reorganize the entire team.

The phrase goes back to at least 1451 in French. In 17th-century military usage, a carte blanche was a blank signed document handed to a defeated party, letting them write their own surrender terms. Over time, that empty page came to mean unlimited power to act.