Atletico Madrid are confident that an email can prove that Antoine Griezmann had an agreement to join Barcelona in March, say El Mundo.
The report claims that an email shows an agreement between the player and the Catalan giants in March, meaning that Atleti could take the situation to court.
Barca paid €120m for the French forward after his release clause dropped from €200m on 1 July, but this evidence of an agreement could mean the Madrid-based club are owed the extra €80m.
A statement from Atleti earlier this month alleged that they would take the Catalan giants to court over the deal, which they believe they can prove was agreed before the clause dropped.
“Atlético de Madrid believes that the amount deposited is insufficient to meet its rescission clause, since it is obvious that the commitment of the player and that of Football Club Barcelona was closed before the aforementioned clause was reduced from €200m to €120m,” read the statement.
“It was also prior to the date on which the clause was modified the communication that the player made on May 14 announcing his disengagement from the club.
“Atlético de Madrid believes that the termination of the contract occurred before the end of last season due to events, acts and demonstrations carried out by the player and that is why he has already started the procedures he considered appropriate for the defense of his rights and interests legitimate.”