It clearly states in his contract he can leave at the end of each season for free.... and the end of the season is August this year.
Afaik it mentions a specific date, not the end of the season, even if that's what it's supposed to mean. And here's where the legal battle would be played: Would a court go with the verbatim or with the intention.
Messi doesn't have time for a new project. If he wants more CL and personal glory he needs to move. By 35-36 he'll be finished. He has 2-3 years tops. Barca can't form a team that wins the CL in just 2 seasons from this point.