give someone the green light

(verb) to give someone approval to do something

Example Sentences:

The employee asked the boss to give him the green light to take his vacation in December.

A: I can’t believe that she made this mistake!
B: I know! We’re going to lose thousands of dollars!
A: Who gave her the green light to do this?
B: I have no idea!

Father #1: Are you planning on giving your son the green light to go to Canada to study English?
Father #2: Yeah, I want him to speak well, so I’m going to let him go.