United are reportedly keen on two Sporting Lisbon midfielders - Mane and William Carvalho.
And like Carvalho, who's thought to have a €35m release clause, Portuguese newspaper A Bola reports that Mane also has a massive price-tag inserted into his Sporting deal.
He signed a new Sporting contract in August last year, a five-year deal that inserted a €45m buy-out clause in an effort to warn off interest from the likes of United.
United boss David Moyes has been on an extensive scouting mission this month, taking in games in Italy and France while club officials have been in Spain.
A central midfielder is thought to be his top priority, with the defending Premier League champions struggling in the centre of the pitch.
£27m summer signing Marouane Fellaini is currently sidelined with wrist and back injuries, while Anderson has been strongly linked with a move to Italian side Fiorentina.
Mane, a 19-year-old Portuguese youth international, made his first-team debut for Sporting last year and has since played four league games for the club. But the huge price tag could put United off.
The Old Trafford club have a strong relationship with Sporting representatives, having signed Nani and Cristiano Ronaldo from the club in the past.