"we're not a selling club"
Is just hollow statement, That doesn't stack up in the real world, If a player wants to go, And their is a reasonable offer for him, They'll let him go, Only a lunatic and frankly a stupid person will try to stop it.
Contracts while they 'legally' bind you to a club, It has no authority to force a 'mental commitment' to the club, And even if there was such a thing, There is no tool that can measure it.
If verratti said he wants to leave to juventus, And they're willing to pay 20M for him, We'll expect PSG to say, "No, We'd rather having a complacent player than giving you up for that amount".
But if verratti said he wants to leave to FCB and they're willing to pay 100M for him, Then they'll say yes, And that's reality, No club will rather keep a player not willing to commit and waste earning 100m, Not to mention having a complacent player in the squad is infectious and it'll risk the commitment of others also.
Will verratti have to pull some shenanigans, Like refuse to start pre-season to bend PSG hand, Probably so.
As long as his intent to move is very strong and there is that 100m offer from us, This transfer is happening.