While the Spanish club want him back on loan, other teams are ready to do a permanent deal.
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Per Mundo Deportivo, Chelsea, Borussia Dortmund, Juventus and Napoli are among the teams interested.
United would prefer a sale abroad, but would not object to a Premier League club bidding if the figures were high enough.
Greenwood has not played for United since he was arrested on allegations of domestic violence two years ago.