If they reach an agreement with PSG, they'll need to convince the player. Manchester City is also reportedly interested in signing Donnarumma and wants to sign him. If this is true, Manchester United's situation is very difficult, as City, who play in the UCL, are a much stronger candidate. If Manchester City doesn't want him, perhaps United can persuade him.
In the current situation, Manchester City may have a much better chance of landing Donnarumma than their rivals, because whatever the reason, Donnarumma himself will certainly choose a much better team. Besides playing in the Champions League, Manchester City also offers Donnarumma the opportunity to continue his good form, and Manchester City's great potential to win the Premier League title also makes him attractive. While these aren't the only possible reasons, I see Manchester City's chances as much greater.