The Norwegian joined the Red Devils from Tromso in 2020 and was highly-rated at the club but has not made a first team appearance for the Premier League giants.
Manchester United were keen for the young midfielder to sign a new four-and-a-half year contract at the club, according to the Manchester Evening News, but no agreement could be reached.
The report states that Hansen-Aaroen was not convinced that there was a pathway set out for him to reach the Manchester United first team.
The club wanted him to sign a new deal and send him out on loan, but the Norway Under-21 international was not sure he had much of a future at Old Trafford.
The Telegraph report that the teenager was also asking for far more than Manchester United were willing to offer in terms of wages.
Hansen-Aaroen felt both his wages and chances of first team football would be improved away from the Red Devils and the club has decided to cash in while they can.
The fee is undisclosed but the deal reportedly includes add-ons, a sell-on fee and a clause allowing Manchester United to match any future bids for the youngster.
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