I'm no expert in law, but as a Spanish person that is familiarized with Spanish league and has been following but Spanish league and Aday Mara since he was playing for Zaragoza, let me explain what I know about the case.
In Spain there is no high school/college basketball, the profesional clubs have teams in lower categories so they develop players that later can go to the pro team.
Mara was in Casademont Zaragoza project and signed with the club a contract when he was 16 when he was still a raw prospect an increasing contract up to 2027 in which the first two years he only got paid 40,000€ combined (allegedly, it's not confirmed the exact amount), he started quickly playing in the pro league where teams like Real Madrid and Barcelona has a lot of NBA players earning millions, so he was really underpaid.
So when he turned 18 in 2023 and got offers from NCAA, he decided to break the contract and go to UCLA, so he was supposed to pay a release clause of 700,000€ to the club. He refused to pay alleging it is an abusive clause so the legal issue started then. I don't know the details of the case but this is probably going to be resolved in court.
As It was known today, Casademont Zaragoza supposedly reached the NBA and OKC about the issue demanding the payment, threatening to keep Mara from playing (i don't know what kind of authority they have for that) and as they got no response, they made It public today. I think this move is trying to put pressure on Mara and the Thunder to make the payment by getting the NBA and the USA media involved.
Don't take this as the 100% truth, it's just what I got from following Aday. I hope he can play with no issues and have a role on the team.