Just because LaLiga doesn’t accept the contract doesn’t mean it’s invalid. Both have signed, it will be at least a legal mud fight if he wants to change for free.
But I assume that this is only theoretical, that Gavi wants to stay and fulfill the contract (Barca too) and that LaLiga is playing some power games with Barcelona again.
No, not really… in principle “pacta sunt servanda” certainly applies, however with restrictions, the contracts must not be illegal or immoral. As an example, your employer could also write you in the contract that you have 10 days of paid vacation and, should the contract be checked, this passage will be deleted despite your signature and consent, since you are legally entitled to 20 days if you start from Germany . That would be an example of illegality.
It would be immoral if I bought your brain from you for five Neptunian dinars and you signed this sales contract, since I could insist on your brain here, but you would not be able to see the consequences if you signed such a contract and I therefore take advantage of your position.
In this case, the clubs have committed themselves to participating in a league to a common code that is intended to guarantee fair competition and voluntary cost control. If the league, as the supervisory body, is of the opinion that this contract violates the imposed regulations, there are really only two possibilities. The club leaves the league and insists on the validity of the contract, or the club accepts that it has violated voluntary self-regulation and cancels the contract to comply with the self-imposed rules.
The third possibility would of course be to challenge the self-imposed rules at a higher level in order to have them recognized as illegal or immoral. However, this will also be difficult because the freedom of association would be called into question here and, after all, no one is forcing you to be a member of an association.
In the end, we currently have Schroedinger’s Gavi, whose contract ends on June 30th, so that he can negotiate freely with any other interested club, and on the other hand has already signed a contract that has been suspended in the best case, and canceled in the worst case.
If I were Gavi, I would walk a long way very quickly, especially when I see how my (ex) colleagues are treated… I heard there is a Catalan in Manchester…. (even if I don’t know the club at all like)