Sir Alex Ferguson watched Viduka during Leeds' 1-1 draw at Manchester City, with the striker scoring his second goal in as many games to give Leeds another vital point in the battle to beat relegation.
"No club can say a player is not for sale if the figure is not the right one," added Birch, with Man Utd understood to have already made a tentative approach for the Aussie.
"But we don't want to sell anybody and we don't need to sell anybody. Selling a player for £3.5m makes no difference to the level of debt and only weakens the playing squad."