Regardless of the legal argument Barca may bring up, they aren't doing themselves any favours with this stance.
The way I see it, Bartomeu has a vested interest in getting a fee for Messi, you could argue he has engineered the whole situation what with the social media campaign they run against him.
Bartomeu is on the hook for any financial losses that Barca make at the end of his presidency, it's shouldn't come as a shock to anyone to think he see's Messi as a way of resolving that.
Whatever Barca try and do, if Messi has made up his mind, there's only one winner, Messi.
Even if they prevent him from leaving this season, he can still leave at the end of it, I would imagine it will make him more determined than ever.
Barca the club/the organisation are damaged either way, even with a change of board members!
The last thing you need in any organisation is a disgruntled employee, the unrest will spread like a cancer....
Oh and Tebas, well he's just a bitter c^%t!