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.
Since its inception, Brighton has been known for producing quality young players. At the Brighton academy itself, this has become an investment tool. Buying seeds all over the country and turning them into quality young players who are ready to be brought in by big teams. That's how Brighton is, they might be said to not be aggressive in winning trophies, but players who have graduated from Brighton can show their results at the club they occupy.
Regarding Mitoma, I think the same thing applies, it is possible that after his contract is finished, it could happen that before his contract, a big club will be interested in buying him. Mitoma is a talented player and he needs to develop at a club that can achieve success in winning trophies. Therefore Brighton remains open and losing Mitoma in 2025 will not cost them anything. In fact, Mitoma will be profitable because it was bought at a cheap price, then trained until it becomes a regular market player, its market price automatically rises drastically. Brighton is a business club so I wouldn't be surprised if he ends up being sold.
They are getting the fruits of their labor with these good seasons. Sure they can't spend as much as some other teams, the top ones will always overspend brighton every single season, they are just not that much famously known, hell United has been terrible for a long time and haven't won a single thing and yet they are still making more money than most other clubs, except maybe a few.
However, with Brighton having this type of academy, it proves that they could get some great young players, sell some stars, and the rest could go to lower leagues for a good chunk, maybe 1 million pounds is not a lot, but if you can sell 5 of them to lower leagues every season that would be 5 million income from just academy investment. They grew bigger and bigger and built the club into something that is much better.
Nowadays I am expecting them to do it on the main squad too, get young players and make them play premier league quality minutes and then sell them for a lot more money, like 10-20 million profit from each, which should give them a consistent amount of time to get better.