Earlier this month, the Catalan giants saw Neymar depart for Paris Saint-Germain, who met the €222million release clause in the player's contract, smashing the world transfer record in the process.
Neymar has made a stunning start to his time in Ligue 1, scoring three times in his first two games, but on Tuesday Barca released a statement confirming they were suing the forward for at least €8.5m.
Barca's complaint relates to Neymar having signed a fresh deal until 2021 in October last year, with the club hoping to recoup his "renewal premium" as well as €8.5m in damages and a further 10 per cent in arrears.
The club went on to say that if Neymar is unable to pay, PSG will be expected to cover the costs.
"In this lawsuit, the club demands from the player the amount already settled in relation to the renewal premium for a breach of contract; €8.5m in damages; and an additional 10 per cent in arrears," the statement read.
"The club also asks that Paris Saint-Germain assumes the payment of these amounts in the event that the player is not able to take charge.
Shortly after Neymar completed his move to PSG, Barca announced that the player would not receive a €26m loyalty bonus after they accused him of breaching his contract.
It was reported that Neymar's father - also his agent - held up the move to PSG beyond the end of July in order to trigger the payment, which had been inserted as part of his new deal.
"There were three conditions [to the bonus]: one, that the player does not negotiate with another club in the last three months before July 31," Barca said at the time.
"Two, that he will express publicly his commitment to fulfil his contract.
"Three, the collection was to be made on September 1, to ensure that he did not go to another club."
After scoring twice on his home debut for PSG - a 6-2 thrashing of Toulouse on Sunday - Neymar took aim at the Barcelona board, claiming "there are directors that shouldn't be there".
"I had four very happy years there, but Barca deserve much better. I have a lot of friends there," he added.
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