I’m not sure if it’s really possible to have significant discrepancies between declared income and observed spending patterns without prompting an audit or investigation. You can buy like an expansive pc with crypto without the RSI noticing it, but buying a €565k house with 3000 XMR you bought in 2015 at ~$0.5 without the need to explain the origin of the funds seems unlikely.
Discussion
First I wouldn't buy a house for that price, it is just overpriced and not at a place I'd enjoy living. And the second part is that you don't need to declare the real price you pay.
Maybe in your country this would be a problem, around here it isn't. The only sideffect is that legally you have to offer the neighbors the chance of buying a house/land for the public price you are selling.
I live in Portugal too. I see your point, but underdeclaring property prices seems risky. Beyond the tax implications, it could draw BCFT scrutiny, especially for large transactions. Paying a significant amount under the table or using untraceable funds like Monero might raise red flags with notaries or registrars, who are obligated to report anything suspicious. And wouldn’t the IMT and stamp duty still need to align with the official declaration?
Will answer through a different way.