Is there anything stopping a team from offering Roki Sasaki an extension immediately after he signs?

I know the value of getting Roki is that he’ll be cheap and controllable for the first 6 years, but why wouldn’t a team (the Dodgers) just tell him that if he signs with them, they’ll just immediately extend him. Couldn’t they just use this as a sales tactic to sway him and get him to sign an extension before he even pitches an MLB game kind of like Jackson Chourio. I know we keep thinking he’s not it in for the money, but what if he actually is…