Barcelona are seeking at least 8.5 million euros ($10 million) in compensation from Neymar for breach of contract, the club said on Tuesday as the bitter fallout rumbles on from his world record 222 million-euro move to Paris Saint-Germain.
The move comes after Barcelona refused to pay a separate 26 million-euro bonus due to Neymar for signing a five-year contract renewal last year.
"In the lawsuit, the club demands the player return the already paid sum for his contract renewal as he has not completed his contract; 8.5 million euros in damages; and an additional 10 percent because of delayed payment," Barcelona said in a statement.
Neymar's family hit back on Tuesday in a statement released by the firm representing their interests.
"This news was received with surprise given that the player fully respected the contract valid at the time," said NN Consultoria.
"As for the bonuses owed for the signature of the contract in 2016, stipulated in the contract and not paid by FC Barcelona...(Neymar) has already started formal proceedings to recover the money before the competent court."
Despite being under contract, Barcelona were powerless to stop Neymar's move to PSG earlier this month as the French side met the buyout clause in his contract.
Barca added the demand was lodged against Neymar on August 11 with Barcelona's employment court, eight days after his stunning move to Ligue 1.
The Catalan giants have also passed the claim onto FIFA and the French Football Federation via the Spanish Football Federation to take what they see as "the appropriate legal effects".
Moreover, Barca urged PSG to pay the amount they are seeking if Neymar -- who is reported to earn 30 million euros a year after tax for the next five years in Paris -- isn't able to meet their demands.
PSG responded on Tuesday evening by saying they were "surprised" at Barcelona's move.
"Paris Saint-Germain reiterates that, like Neymar Jr., it has always respected all applicable laws and rules in its dealings and, once again, regrets the attitude of FC Barcelona," read a club statement in English.
Barcelona's latest move comes two days after Neymar fiercely criticised the club's under-fire board after scoring twice in a stunning home debut as PSG beat Toulouse 6-2 on Sunday.
"There are people in charge that shouldn't be there," Neymar told reporters.
"Barca deserve much more and everyone knows that."
The split between Barca's top players and board was evidenced as a smiling Neymar posted photos on his Instagram page of the Brazilian visiting former teammates including Lionel Messi, Luis Suarez, Gerard Pique and Ivan Rakitic on Tuesday.
"Friends that I missed," Neymar wrote alongside a picture of him being embraced by Messi and Suarez.
Messi is yet to formally sign a new contract, the terms of which were agreed back in July, provoking speculation over his future.
The case is just the latest in a series of legal disputes that blighted Neymar's four-year spell with Barcelona.
Barca president Josep Maria Bartomeu, former president Sandro Rosell, Neymar and his parents are to face trial for fraud and corruption in Spain over the murky transfer that brought the player from Santos to Barca in 2013.
Investment company DIS -- who owned 40 percent of Neymar's sporting rights at the time of the transfer -- believe they were cheated out of their real share of the deal.
Barca also settled a case with the Spanish tax authorities over the transfer for 5.5 million euros last year.
Bartomeu is under mounting pressure as Barca have so far failed in their attempts to replace Neymar with bids for Liverpool's Philippe Coutinho and Ousmane Dembele of Borussia Dortmund rebuffed.
Following Barca's 5-1 aggregate thrashing by Real Madrid in the Spanish Super Cup last week, the hashtag "bartomeudimiteya" (Bartomeu resign now) was a trending topic worldwide on Twitter.