Not within 2 years. He would still only be 23 and Messi more importantly only 29.
3 years then it becomes an issue. Beyond a real danger which is why the length of the buyback is almost more important than the amount.
I also don't think if he's doing that well he would leave and screw us and Martinez. If he goes back to Barca it will be when he's around 25 so I don't think it's much of an issue.
Unless he smashes in 30 goals next season and Messi gets injured.
Its his boyhood team, one of the biggest teams in the world, if they come asking he will go.