The 20-year-old, who can play at left-back or centre-half, is out of contract at the end of the season, although Walsall would still be entitled to compensation if he moved then due to his age.
Saddlers boss Paul Merson has said: "I have had a call from a club asking about Julian and how much we wanted for him.
"I told them the figure. It is unbelievable money."