Barcelona are pursuing damages from Neymar for what they allege to be a "breach of contract".
The Brazil forward left the Nou Camp this month for a world-record US$332 million, joining Paris Saint-Germain in a move that more than doubled the previous highest transfer fee.
Life as a PSG player has started well for the 25-year-old, who scored on his debut and then added two more along with three assists in a stunning show against Toulouse on Sunday.
Relations with his former employers, however, have turned sour.
On Monday Neymar said Barcelona deserved "much more" than the current board of directors and added that it made him "very sad".
On Tuesday, the Catalan club said they were chasing EUR8.5m (NZ$12.6m) in damages with a 10 per cent addition and, that if Neymar could not pay, PSG should.
The demand appears to relate to a payment made to Neymar when, in 2016, he renewed his contract with Barcelona through to 2021. It was sent to the Spanish football federation on August 11 with a wish for it to be forwarded to the French federation and FIFA.
"Barcelona decided to take these actions in defence of their interests, after a unilateral termination of contract urged by Neymar Jr a few months after signing the renewal until the end of the year 2021," a statement read.
PA
