Marseille has an obligation to acquire a center-back for £5.1 million, subject to appearances and Champions League qualification; United is in preliminary talks with Newcastle about a move for goalkeeper Martin Dubravka.
Marseille will pay the entire salary of the 28-year-old Ivory Coast international while he remains at the Stade Velodrome.
The Ligue 1 club is obligated to sign Bailly permanently in the summer of 2023 for £5.1million (€6m), subject to appearances and whether Marseille qualifies for the Champions League.
Jose Mourinho brought Bailly to Manchester United from Villarreal in the summer of 2016 and the center-back went on to make 113 appearances for the club, scoring once.
He helped the club win the League Cup and Europa League in his first season but he has also been hampered by injuries throughout his time at Old Trafford.
The signing of Lisandro Martinez this summer saw him fall down the pecking order with competition at center-back also coming from Harry Maguire, Raphael Varane, and Victor Lindelof.
A statement from United described Bailly as a “popular member” of the squad and added: “Eric has our best wishes for the campaign to come and we will be following his progress closely.”
Manchester United has held initial talks with Newcastle United over the signing of goalkeeper Martin Dubravka.
Dubravka has lost his No 1 spot following the arrival of Nick Pope and wasn’t in the squad for Wednesday’s EFL Cup tie at Tranmere.
United is in the market for a ‘keeper to provide competition for David de Gea and Tom Heaton after Dean Henderson joined Nottingham Forest on loan.
It is thought Dubravka would be open to the move.