I'm a tax advisor for a large reputable firm, and a few years me and lots of my colleagues got loads of messages from randoms on linkedin about opportunities at various "boutique firms focussing on high net worth (HNW) individuals".
Basically, if I'd got the job, I'd have spent my days setting up trusts and looking for loopholes to help billionaires avoid paying their income/capital gains/inheritance tax, legally or otherwise.
Sufficed to say I didn't bother replying.
So I have no doubt whatsoever that Messi and Ronaldo were told by someone that if they signed here, pay me "x" million euros, and it'll save you 100 times that in taxes, and they just did.
The reason that the taxpayers are on the hook themselves rather than the partners, is because the slippery partners will have included a million and one caveats, which basically exempts themselves from prosecution if any of their "advice" ends up beings considered illegal by the various authorities....