this combined with an untraceable payment seems to me the less risky case. for me it looks like in such a case its word against word even if the receiver might be pressured by the tax office. the only link could be to have a look if the person giving the money has withdrawn cash or crypto around the amount stated by the receiver. lets presume you can not find any trace of these withdrawl at the person giving the money.
or is my understanding completely wrong
@Sols ?
It's all speculation, so it's hard to say. One thing all busted tax evaders have in common is they underestimated the tax authority.
Let's say the tax authority (you mentioned it being a high tax and skilled EU tax authority) is offering your partner a deal where they only have to pay taxes and a minimal fine and not spend any time in prison, if they give you up.
The person squeals and says the payments came from you. They provide all the information they can: emails, chat messages, call logs, your contact details, anything it takes to make the tax authority go away. Faced with prison time and/or life-long debt, people get desperate.
What happens next depends on what evidence is made available to the tax authority and how they process it.
They know you're going to deny, so they probably won't even bother asking you. They'll just get all the information they can about you, starting with your tax returns, your bank accounts, and your travel records. They'll start piecing together something and look for anything that could explain how you were able to pay the partner using anonymous payment method.
If you paid your partner in XMR and the tax authority finds you sent money to or from a crypto exchange, that's an opening to get your details from the crypto exchange. They don't need all the evidence. They just need a little bit. Then they can send you a letter asking for all your crypto wallets and any other information they want that you don't have.
But let's suppose you obtained the XMR not via an exchange. The tax authority will look into how you have been spending your money. If they find something that looks like it could be a large cash withdrawal or wire transfer for the purpose of buying crypto anonymously, they have found their way in. Suppose you withdrew several thousand EUR at multiple occasions. Why? What was the money for? They'll ask you for receipts of what you spent that cash on.
Whatever you think sounds unreasonable and too invasive, the tax authority has already been there and done that.
If at some point, they have enough evidence to satisfy their own internal guidelines, they'll send you an invoice for taxes they think you owe. Then you have 30 days to pay the invoice. You can begin your appeal process, to prove your innocence before a court of law. The tax authority might also pass you on to police or prosecutor (depends on local law), so they can investigate/prosecute you for money laundering.
That is if they care and if they catch you, though. It might never happen. A lot of people get away with petty amounts. For larger amounts, though, it's only getting harder and harder...