"Ronaldo is a Real player and will continue to be so as far as we are concerned", Perez told Marca sports daily Tuesday.
The cases are about the profits made from image rights, not salaries from their clubs.
Madrid were on the verge of bringing former Atletico Madrid goalkeeper De Gea back to Spain in 2015 but the move, which included sending Real custodian Keylor Navas to United as a makeweight, was derailed as the clubs failed to close the deal before the transfer window closed.
Both Ronaldo and Mourinho are represented by Portuguese agent Jorge Mendes.
"Ronaldo took legal advantage of what is known in Spain as "the Beckham law" - which allows foreign sportspersons to pay 24.75 per cent income tax, rather than the 48 per cent applied to Spanish nationals".
Like many others, the investigation into Mourinho focuses on image rights, adds the paper, with the current Man Utd boss accused of trying to gain an "illicit benefit".
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"The Spanish Government in turn, through the Tax Department, issued a certificate in which it attested that he had regularised his position and was in compliance with all his tax obligations".
Last week, Cristiano Ronaldo was accused by prosecutors in Spain of defrauding Spanish authorities of $16.4 million in tax between 2011 and 2014.
He will appear in court at the end of July to defend himself, although there is no formal charge at this stage. Ronaldo via his representatives last week denied any wrongdoing.
The 54-year-old Portuguese, who coached Real Madrid from 2010 to 2013, was accused of "two offences against the public treasury", the office said in a statement.
Real Madrid president, Florentino Perez, has said Cristiano Ronaldo is not going anywhere this summer and that they have received no offers for the player.
In 2016, soccer giant Messi, along with his father, was convicted of three counts of tax fraud equaling some $4.6 million earned from the use of Messi's personal brand.
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