The Red Devils are scrambling to sign a right winger after failing to land priority target Jadon Sancho from Dortmund.
Sarr, 22, joined Watford for £40m last summer and he was one of the few players to impress as the Hornets were relegated to the Championship.
With a deal for Sancho dead in the water this summer, United still plan on signing the 20-year-old England international next summer.
They have therefore tabled a season-long loan deal to Watford for Sarr and offered to sign the winger for £45m at the end of the campaign if they wish to do so.
However, United’s plans are quite clear in that they will try to sign Sancho once again in 12 months time and Watford were unwilling to accept the deal, according to Telefoot Chaine.
The Hornets are suspicious of United’s true intentions at the end of the season and suggested that the Red Devils pay a £25m loan fee if they fail to take up their option to buy the winger.
United, of course, refused that demand as they’d essentially be paying £25m to have Sarr for one season.
It remains to be seen with the deadline closing in whether United will go back with another offer for Sarr but the club have more time to sign the Ivorian given Premier League clubs can sign Championship players until October 16th.
However, the deadline to register players for the Champions League is tomorrow.