Barcelona are to sue former player Neymar for £7.8m over an alleged breach of contract in the wake of his £198m move to Paris Saint-Germain.
The amount is understood to relate to a loyalty bonus Neymar received after signing a contract extension in October 2016.
"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.
Brazilian striker Neymar said he was surprised by the decision but warned he is not going to give in.
"It is noteworthy that this news was received with surprise, since the athlete completely fulfilled the contract then in force, with the full deposit of the amounts set out in the buy-out clause freely agreed with F.C. Barcelona for his release," Neymar's representatives said in a statement.
The statement added that the 25-year-old forward's lawyers were preparing a formal defence to the demands.
Barcelona have passed the complaint to the Spanish football federation (RFEF) after first filing the lawsuit in court, as well as demanding PSG subsidise it, should Neymar not be able to take responsibility for it.
PSG released a statement later on Tuesday, expressing surprise at Barcelona's action against Neymar.
"Paris Saint-Germain was surprised to learn of FC Barcelona's statement today announcing the start of legal proceedings in Spain against its former player Neymar Jr. and indicating that should Neymar Jr. fail to pay the monies claimed by his former club, Paris Saint-Germain will be held responsible to pay the outstanding sum," read a club statement.
"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."
Neymar joined PSG on a five-year contract on August 4 following a protracted transfer saga.
The Brazil international's move looked set to be held up after La Liga initially refused to accept the funds for the £198m buyout clause.
The deal finally went through and the 25-year-old scored on his league debut in the 3-0 win at Guingamp.
He also scored twice in the 6-2 victory over Toulouse that leaves PSG top of Ligue 1 on goal difference but the fallout from the transfer looks set to rumble on.