The striker's deal from Shanghai Shenhua ends on May 31 and the Chinese Super League club have been reluctant to extend it, it is understood.
United are keen to keep him for the rest of the season, with the Premier League intending to resume next month.
While the deal is not completely dead, Shanghai have not been open to agreeing to a new one with their season also due to start in June after it was delayed from February due to the coronavirus pandemic.
Ighalo scored four goals in three starts and made eight appearances in total since he was drafted into Ole Gunnar Solskjaer's side, during the January transfer window.
He joined as cover for Marcus Rashford, who was sidelined with a back injury.
United remain in the UEFA Europa League, the FA Cup and are fifth in the Premier League, three points behind fourth-placed Chelsea with nine games remaining.