Not to stir up controversy, but every football fan not living under a rock are hearing about the exorbitant prices being bid for Mbappe in the newspapers and Real Madrid, being the most probable destination for him, are certain to pay a huge amount of fee to get him from Monaco(more than 100 million euros).
Could it be that Cristiano has realized that this is his last chance to leave a lasting impression in the Transfer Market, since his value will start declining soon. If he leaves Madrid, he will for certain not only break the current record but also, he will fetch more than Mbappe for sure and ensure that his transfer record, just like his goal records won't be overtaken quite easily.
With the size of the ego he has, he could just be using this tax controversy as an excuse to ensure the above scenario.