If the player wants to move they have no option but to sell, for what price is another story.
No, he's under contract, PSG can definitely keep him for at least one more year, maybe two depending on EU law if they really want to. Of course they could decide that it's better to let an unhappy player go and use the money elsewhere. With Ancellotti and Leonardo already gone, maybe Blanc would want a different project.
There's also a human element. This is possibly Thiago Silvas last chance for a big move since he is nearing 30. He wanted to stay at Milan but he acquiesced and signed a contract with PSG, in the process helping them in the Champions League and also winning the League. Letting him go would be a way to pay him back for the good he has done for them. Treating players badly also doesn't do the morale of the rest good either.