It turns out that Chelsea own Mourinho's name and image rights and now want Man United to pay a hefty price for them.
Jose Mourinho's dream managerial move to Manchester United has stalled two days after it was reported a formal agreement had been reached.
A small matter is slowing down the process: Chelsea 'own' Mourinho.
It turns out that Mourinho's former club owns his name and image rights and although the trademarks are limited in scope, Man United may have to shell out a large sum of money to resolve the issue.
The 53-year-old Portuguese was set to take over as Man United manager after Dutchman Louis van Gaal was sacked on Monday.
But according to media reports Chelsea are demanding United pay a sum running into millions of pounds for three separate trademarks of Mourinho's name, one of which the club have owned since 2006 during his first spell in charge.
The unusual scenario has baffled experts who have questioned why Mourinho apparently signed over his own image rights, which are a huge driver of revenue.
Sports rights lawyer Julian Moore told Reuters, "It is pretty uncommon for a manager of his stature not to have full control over the rights to his name and image, and pretty rare for a club to retain rights independently of a manager."
Chelsea believe they are entitled to compensation for allowing United to market their new manager on specific items.
A similar conflict arose when Mourinho first left Chelsea in 2007 to join Inter Milan and then Real Madrid, but the London club allowed their former manager's name to be used by those clubs under licence.
The Times reported that Chelsea had two further trademarks on Mourinho granted by the European Union's intellectual property office in 2013.
Although owning his name is now of little value to them, the club are believed to feel a direct commercial rival like United should pay for the privilege of using it.
The subject has formed part of extended negotiations between United Chief Executive Ed Woodward and Mourinho's agent, Jorge Mendes.
Also requiring resolution are potential conflicts of interests between United's extensive list of commercial sponsors and Mourinho's personal endorsement deals, with cars, casinos and watchmakers reportedly requiring clarification.
Moore said, "United will want to ensure they have cleared all rights because they have a complex web of interests around the world. They will want all rights sorted before hiring someone of this type."
United and Chelsea did not respond to requests for comment on Wednesday.