The transfer market has been hit with a trigger following Ronaldo's hint that he could leave Spain if he is eventually hammered in the tax case. A claim for the money has been presented at a local court, reports the Daily Mail.
Another Mendes client, Colombian striker Radamel Falcao, is suspected of failing to correctly declare 5.6 million euros of income earned from image rights between 2012 and 2013 while he was at Atletico Madrid.
The 54-year-old Mourinho was Real Madrid coach from 2010-13.
This news follows reports of Madrid star Cristiano Ronaldo potentially facing tax fraud charges himself.
The former Real Madrid boss has been accused of defrauding tax authorities to the tune of €3.3million by a Spanish regional prosecutor, with a complaint filed on Tuesday.
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"As a Manchester United fan I'd love to see him go to Man United".
The football world in Spain has been rocked by charges of tax evasion or corruption pressed against some of its other leading stars.
And after a previous move for the player collapsed, United do not wish to become involved in another lengthy and potentially fruitless pursuit.
Mourinho's management team - Gestifute - and Real Madrid did not immediately respond to CNN's request for comment. The two were given 21-month jail sentences but neither is expected to serve prison time since it was a first offense and the sentence was under two years.
Ronaldo denies any wrongdoing but may still reluctantly pay-up in order to avoid any subsequent fines.
Barcelona's Lionel Messi was sentenced to a 21-month jail sentence and 2.09 million euro fine previous year for tax fraud.