Valencia, AC Milan and German champions Werder Bremen have all been linked with Viduka, but his current salary is a sticking point in negotiations.
Stephen McPhail could also exit Elland Road if Barnsley can put together a package to appeal to both the player and Leeds.
McPhail has another year remaining on his contract and The Tykes, who are now led by former United chairman Peter Ridsdale, are understood to have made an enquiry about the player's availability.
Ian Harte is another player being linked with a move away and Italian outfit Siena, now assured of Serie A status, are ready to renew their interest.
Siena offered £1m to take Harte on loan in January with a view to a permanent move but the offer was turned down.