I think it is more like PSG don't want to sell to Barca rather than PSG trying to keep him against his will. PSG are trying to build a team good enough to compete against the UCL elite, and selling their best midfielder-if not their best player- to the team that just beated them 6-1 in the competition is a backward step. If it was, say, Liverpool, Tottenham or Sevilla level of team trying to sign Verratti and he says he want to play for them, PSG won't have much problem selling him for the right price. Won't be great for PSG's ambition strengthening a direct UCL rival(Barca desperately need a verratti-type player) while simultaneously weakening themselves.