The substantial offer for the Northern Ireland international is understood to include several performance-related add-ons to sweeten the deal.
The Yorkshire club are determined to add immediate quality and depth to their squad as they prepare for another promotion push.
Charles has also attracted significant interest from Manchester United sporting director Jason Wilcox, who previously signed the player while working at Manchester City.
Top-flight rivals Everton and Italian side Atalanta have also closely monitored the progress of the talented young midfielder over recent months.
The 22 year-old player experienced heartbreak last season when his club was excluded from the play-off final due to the fallout from a controversy.
Elland Road officials have already secured captain Ethan Ampadu to a new long-term contract and want to offer fresh terms to goalkeeper Karl Darlow.
The recruitment team is also actively searching for defensive reinforcements, with Borussia Monchengladbach centre-back Nico Elvedi high on their list of targets.
The club has revived separate discussions with Sporting Lisbon midfielder Hidemasa Morita, who is currently available on a free transfer.
Charles fits the exact transfer profile desired by the hierarchy, who want to recruit young homegrown players with high potential re-sale value.
The tidy holding player has earned rave reviews for his technical ability and physical engine when breaking forward from deep midfield positions.
Northern Ireland manager Michael O'Neill publicly praised the player following an incredibly composed individual performance against France earlier this month.
Michael O'Neill stated that the midfielder's disciplined display against elite international opposition proved he is ready for the top level.
Leeds United are pushing to wrap up the deal quickly to beat off competing interest from top-flight suitors.
The Southampton player is open to discussing personal terms if the two clubs can successfully agree a final transfer fee this month.