The club is demanding Brazilian striker return a loyalty bonus paid after he renewed his Barcelona contract last October, less than a year before he signed for PSG.
The Spanish club also wants an additional 10 per cent of that €8.5m as ‘interest’ they claim is owed due to a delay in repaying them the sum.
According to the club, a lawsuit was first served to the player on August 11 and if Neymar is unable to pay up the sum, PSG should.
A club statement read: ‘FC Barcelona has sent the labour claim it made at a Barcelona labour court on August 11 to the Spanish Football Federation (RFEF) so that they can pass it on to the French Football Federation and FIFA, for legal purposes.
‘In this claim, the club demands the player pay back a renewal bonus for a failure to honour his contract; €8.5m in damages and an additional 10 per cent in arrears.
‘The club also demands that Paris Saint- Germain subsidise the payment of these amounts in the event that the player can not take responsibility for them.
‘FC Barcelona has promoted these actions in defence of its interests, after the unilateral rescission of the contract initiated by Neymar Jnr a few months after the signing of its renewal until 2021.’
The legal battle was made known after Neymar who scored twice on his home debut for PSG on Sunday slammed his former club directors in an interview.
‘When I arrived, everything started well, but at Barcelona, there are directors that shouldn’t be there. I’m very sad with them.’