Fiorentina are prepared to sell Nikola Milenkovic in January.
The Sun says Manchester United have been told it will cost more than £40million to land Milenkovic.
The Italians say they will not do business unless their valuation is met.
But that is £10m higher than United want to pay.
Jose Mourinho made a personal check on the Serbia defender in last week's international clash with Montenegro.
He has been impressed by the scouting reports on Milenkovic, 21, and is mulling over a January bid.
Old Trafford chiefs have been told Juventus are very keen and Fiorentina know a transfer battle leaves them in the ideal position to push their fee.