[newaccount74]

If you live in Austria, the very least the Finanzamt will demand is that you pay 16.7% of the amount as VAT, since you can't prove that the payment is VAT exempt. If you don't declare it correctly from the beginning (eg. because you assume it is a payment from a non-EU country that would be tax exempt) you will face a fine on top of it.

If you get caught up in criminal investigations, eg. because you previously worked for a company that was caught paying people under the table, you will have no way to prove that the donation was legal.

I wouldn't worry about anonymous donations of a few hundred Euros, but a 100k anonymous donation will definitely invite a close look from authorities. In my experience tax audits are stressful enough even if you did everything correctly and have documentation for all payments.