The likes of Paul Pogba and Marcus Rashford have looked like completely different players under the Norwegian interim boss, and it will be fascinating to hear what Mourinho has to say about their dramatic upturn in form.
The fee reflects the slightly-less-than two years Mourinho had remaining on his contract, an improved deal which had been agreed a year ago.
Mourinho's first fully-fledged job as a manager was with the Eagles where he took over from former Bayern Munich boss Jupp Heynckes in 2000.
"There are no issues over Jose's contract with Manchester United", said the source.
Their 2-0 home defeat by Real Sociedad on Sunday saw them slump to fifth in La Liga, 10 points behind leaders Barcelona.
But Mourinho is back in work after agreeing a deal with beIN that will see him cover Premier League and Asian Cup matches for the broadcaster.
Speculation is rife over the Real job at the moment, with Santiago Solari struggling for positive results in the wake of Julen Lopetegui's departure in October.
He said on the prospect of returning: "They are things that are said and I will not feed". I don't really want to go there, but the best way to respect a great club [Benfica] and a great president is to say that I was not offered.
The source added: "There has been no contact, but they did get in touch with Jose after Lopetegui was dismissed, but he told them he was the manager of Manchester United and totally committed to the club".
Benfica sacked Rui Vitoria as manager last week, leading to reports Mourinho was under consideration. They're not signing someone they don't know, they are going for someone they know, someone who has already been there.