The Frenchman is ready to leave Old Trafford this summer after reportedly growing frustrated at a lack of first-team football.
The 22-year-old will have a year to run on his contract at the end of the season and, as he is refusing to sign an extension, United need to sell him now or risk losing him for free in 12 months.
Juventus are one of those interested in Martial, but he is also being chased by Chelsea too.
No formal offers have been made yet but, according to Corriere dello Sport, Juve are prepared to pay £44m for the forward.