There was an article in french media and it is true.
The player can ask the FIFA to activate this clause.
However, the sum would have to be paid in one lump payment.
If the buying club has instigated the player demands, it will face sanction too.
And it is rarely used because there is a gentleman agreement between clubs. This clause is the Pandora box. Let's say Barça uses this method. Other clubs could do the same to them and ignore their absurdly high release clauses written in the players contracts.
This is why it is rare and very dangerous to use it.
Dangerous for PSG, not so for Barca.
Most players dream to play @ Barca and stay at Barca. that now includes Neymar.
Look at Raktic, Dembele, Luis Suarez, and even Coutinho before his loan.
Only a few players (Rhafina, Semedo, Dennis Suarez) asked to leave ONLY because of lack of playing time.
Neymar asked to leave because of Messi's shadow and PSG's huge salary. don't think the huge salary alone would have been enough for Neymar to switch to PSG.
Verrati, Rabiot and now Neymar.
As mentioned before, most future stars (besides the few mercenaries) will pay attention and be aware of these denied transfer requests.
Not to mention Nasser's recent 300 million ransom demand.