Mainoo is keen to stay at Old Trafford, but wants a salary that recognises his value to the team.
It's understood that he's demanding a salary in the region of £150,000-a-week.
He is, however, wanting to extend his stay with his boyhood club.
Chelsea have been monitoring his situation with a view to a potential move.
For the Red Devils, the midfielder's future represents a potential opportunity to cash in.
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United know if they sell Mainoo this summer it would help with PSR concerns.
The Red Devils are willing to reluctantly sell homegrown players in order to comply with financial rules.
Man United ended last season with losses of £113.2million but are understood to be confident they remain compliant with the regulations.
This is due to being an academy graduate which mans that any fee received would represent 100 per cent profit.
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Products of the club's youth system would be sold for pure profit having been developed internally rather than bought from elsewhere.
Marcus Rashford and Alejandro Garnacho also falls into the same group.
Mainoo emerged as one of the Premier League's breakout stars last season, establishing himself at the heart of the Red Devils' midfield and earning a spot in England's Euro 2024 squad.
Mainoo's rapid rise from promising young talent to international starter saw him nominated for the 2024 Golden Boy award, with the youngster tipped to continue his strong form this term.
However, the Englishman has struggled for consistency, with injuries and positional changes limiting his impact.
This term Mainoo has been limited to just 15 Premier League starts, while making 25 appearances in total for the Red Devils.
Having picked up an injury in training last month, Mainoo is expected to be ruled out until April.