Press Association Sport understands United made a bid for the Senegalese 23-year-old last week.
Mane enjoyed a successful year at Southampton last term but United's interest in the player was unknown until now.
Southampton signed Mane from Red Bull Salzburg last summer for £10million.
In his first year at Saints he scored 10 goals in 34 appearances, including the fastest hat-trick in Premier League history in the 6-1 win over Aston Villa
Source : PA
Source: PA