Brighton is seeking approximately £120m for the 21-year-old, with United exploring payment structures, including an initial £80m plus add-ons.
United's manager Ruben Amorim is keen to sign a midfielder to accelerate the team's rebuild, identifying Baleba as a key target due to his profile and potential.
Manchester United would be willing to send a player such as Jadon Sancho the other way, but it is not seen as likely that Brighton would go for that.
Adam Wharton of Crystal Palace is considered a primary alternative, though he may prove even more costly, while Morten Hjumland and Ederson are also being monitored.