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.
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.