The 23-year-old, who has scored 20 goals for the club this season, has reportedly been the subject of two bids from Italian side Catania.
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."
Russell's current deal expires at the end of the season.