Antoine Griezmann's move to Barcelona could be blocked in a row over the fee, La Liga president Javier Tebas says.
Atletico Madrid complained after Barca announced they had signed the France forward, having activated his 120m euro (£107m) release clause on 12 July.
The clause dropped from 200m to 120m euros on 1 July, but Atleti claim the deal was done before then and that they are due the larger amount.
Griezmann has already played a friendly for his new club, a 2-1 friendly defeat by Chelsea in Japan.
Tebas said: "[Atletico] submitted a complaint and put in doubt whether we should pass Griezmann's licence to Barcelona.