Mitoma is an important palyer in Brighton and his effect on Brighton is the reason to say Brighton won't lose this player easily.
However, Manchester United is not in a good situation and there is a lower chance for Mitoma to accept this offer if Pep Guardiola sends him an offer at the same time. The contract with Mitoma will expire in 2025 and if he continues his good performance he can even be linked to other teams.
I have doubts about this, because you can see previously how Brighton is happy to let go of their players if someone wants to offer a high price. We saw how they let go of Alexis MacAllister and Moses Caicedo because both were bought at high prices by Liverpool and Chelsea, if the two teams from The city of Manchester could make a tempting offer, I'm pretty sure Brighton will sell him without thinking that they need Mitoma who has impressed for several seasons with Brighton. Could Brighton be like Dortmund? who sells his potential players to make a lot of money in order to stay safe in the English premier league, because a club needs a lot of funds to maintain its performance and they will look for other young players who have good potential.