Barcelona believe the only way Lionel Messi can legally leave for free is if he commits to not playing football next season, sources have told ESPN.

Barca's interpretation of a much-debated clause in Messi's deal is that he could quit the club but would have to forgo his salary for a season and could not sign for a new club until next summer, when his existing terms are due to expire anyway.

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Messi, 33, is attempting to exercise a clause in the contract he signed in 2017 that allows him to leave for free at the end of each season. Sources at Barca have told ESPN that the deadline for activating the clause in order to sign for another side expired in June.

Messi's legal team continue to argue that he has already unilaterally rescinded his contract with the club. For that reason, the forward didn't turn up for the start of preseason Sunday, as it would have compromised that claim. Barca returned to preseason training Monday without the club captain.

Meanwhile, reports in Spain over the weekend claimed Messi's €700 million buyout clause is no longer active, causing further confusion.

Barca insist that is not the case and La Liga agreed with them on Sunday, saying they have had access to the contract. In a short statement, the league said Messi's deal is still active and they would not facilitate his registration for another club without the clause being paid.

According to several legal experts consulted by ESPN, the clause in Messi's contract can be interpreted in a variety of ways. If he and the club cannot come to an understanding, the difference in opinion could yet lead to a landmark court case.

Despite the standoff, sources have told ESPN that Barca's offer for Messi to extend his contract at Camp Nou by an additional two years remains on the table.

Barca president Josep Maria Bartomeu wants to sit down with Messi's father and agent, Jorge, when he lands in Spain this week to sign a deal that would link the player to the club until 2023.

Sources would not reveal the exact terms of the deal to ESPN but did confirm the salary would be similar to what Messi earns, close to €100m gross annually.

However, the club are not optimistic of Messi signing the deal, given his desire to leave the club, with Manchester City still the favourites to sign the six-time Ballon d'Or winner.

Meanwhile, sources told ESPN that Barca plan to reply to Messi's latest burofax, which arrived last week explaining he would not attend preseason, by asking him to reconsider his position on wanting to quit the club as they fight to keep their record goal scorer.