The Red Devils had received no interest from any club for the Frenchman this summer and expected him to stay.
Jose Mourinho had planned to make him their star man and give him more of a leading role at the club.
United instantly rejected the proposal and insisted the World Cup winner is NOT for sale at any price. The 25-year-old's agent Mino Raiola is keen on the deal, and is thought to want to move his client on.
And Old Trafford chiefs, including Mourinho, are understood to be angry with Barcelona's approach.