Gabriel Heinze is understood to have a letter from David Gill, the Manchester United chief executive, that allows the Argentina defender to leave the Barclays Premier League champions if the club receive a bid of £6.8 million. But if Heinze is relying on that letter’s legal force to allow him to move to Liverpool, he may be disappointed. According to a leading sports lawyer, the document may not be legally binding if Heinze has not offered anything in return for his release.

“It all depends on what legal status the letter has,” the source, who asked not to be named, said. “If David Gill has written a letter to Gabriel Heinze saying you can go if we get this valuation, then does that actually vary his employment contract? There is an argument that it doesn’t.

“It would show a clear intention on the part of United that they were willing to let him go for the sum of £6.8 million – if that is the figure – but it would not be unequivocal and it would not prevent Manchester United from arguing that it is just a ‘gratuitous promise’, not a contract. A gratuitous promise is unenforceable in a court of law.

“Basic contract law is offer, acceptance, consideration and an intention to create legal relations. What was the consideration from Gabriel Heinze in exchange for this promise that he can leave for £6.8 mil-lion? I promise to turn up for work and they promise to pay me a certain amount of money, and that is the basic ‘consideration’ – that I will turn up because they pay me. But Heinze is already under contract to turn up for work anyway. What more could he have offered to procure this promise that he can go if someone pays £6.8 million?

“It could come down to a question of integrity and credibility for Manchester United and David Gill if he has signed this letter. It is open to him to say, ‘It was a promise, but I didn’t know the circumstances in which you were going to rely on that, and it is not a contract and you cannot hold me to it.’ ” If, however, Heinze did provide a consideration, United may not be able to prevent him moving to Anfield. “Assuming there is some enforceability to this promise that he can go if they receive a certain offer, then unless that letter stipulates that the player cannot go to a certain country or a certain club, then he can go,” the lawyer said.