As Slav says it happens when the other team lose interest, it also occurs sometimes when the transfer window closes, however, once the interest starts up once more he whines again. Had it happen with other players not just Pogba (De Gea gets grumpy when teams become interested as well and then when no offer appears he is wanting to return to the fold). You just have to stand firm and play chicken with PSG (oh and make sure you renew his contract when he is happy so you don't lose him on a freebie).
I often play with mid table teams and what i do with all player is when giving them new contract i always set release clause to high amount and i always stay on that number, even if they arent happy with rejecting offers lower than clause they step back. There is option in conversation about that clause i use always.
That way i sold Milinkovic-Savic for 200M to PSG. Before that im forced to sell him for less then 90M, same goes with Correa and Immobile