Russell's agent has told United he will not sign an extension to his current deal, which is up at the end of next season.
And Italian Serie A side Catania have failed with two offers for the 20-goal forward, the second of them in the region of £385,000.
Now United have publicly criticised the player's agent, accusing him of being "intent on forcing a transfer, at a fee which falls well below our valuation".
A club statement read: "We have the utmost respect for Johnny Russell and will not stand in the way of him furthering his career.
"However we will not be told by agents how to run our football club. We have invested heavily in our youth set up over the last few years and expect to receive a decent return on this investment when players are sold.
"If we are to continue to invest in our youth system and develop players to enable them to further their careers, we must ensure that this investment is sustainable."