The Portuguese coach, sacked by the Blues in December, has been regularly linked with Louis van Gaal's job over the last three months.
According to a report in Spanish publication El Pais, the Special One will be due £15million in compensation if he is NOT appointed by the club by June 1.
According to the Spanish newspaper on Saturday evening, a source from Jorge Mendes' Gestifute agency - who represent Mourinho - had confirmed that the ex-Inter and Real Madrid boss has already signed to join United, but that the Red Devils can still change their mind.
It's reported that some members of the United hierarchy, including Sir Bobby Charlton and former manager Sir Alex Ferguson, remain reluctant to hand Mourinho the reins at Old Trafford.
“If United do not sign the final contract (with Mourinho) before May 1st, they must pay £5million; If by June 1 he’s still not signed, they shall pay another £10million,”