Barcelona are suing Neymar for a breach of contract after the Brazilian left the club for Paris Saint-Germain earlier this month.
Having already said they would refuse to pay €26 million in bonus payments Neymar was due to receive this summer, Barcelona are now attempting to claim back money already paid to the player.
The Catalan club said on Tuesday they believe they are owed money that Neymar received as part of a renewal bonus when he signed a new contract last year, in addition to €8.5m in damages plus interest.

The amount of the renewal bonus the club are seeking was not immediately made public.
In a statement released on Tuesday, Barca said they had passed the complaint on to the Spanish Football Federation (RFEF), having first filed the lawsuit in a Barcelona court on Aug. 11.
"FC Barcelona have sent a claim to be made against Neymar for breach of contract to the RFEF for it to be passed on to the French Football Federation [FFF] and FIFA," the statement read.
"In the lawsuit, the club demands the player return the already paid sum for his contract renewal because he has not fulfilled his contract; €8.5m in damages; and an additional 10 percent in interest. The club also requests that PSG subsidise the amounts in the event that the player cannot take responsibility."