I am residing in Germany currently.
I registered an account on paymium, which allows withdrawals fortunately as long as the account is verified, which I just did.
But they said that if the amount I receive is too high, they may ask for a proof of origin: i.e. - Notarial act if they were received as donation,
How would I proceed in this case?
If you had not received it there, then you should have consider the options of peer to peer site such as local bitocin, paxful, reminato or any other local exchange that you have there as those platforms you would have less stress in selling for fiat because you would be trading with individuals who are willing to exchange their cash for your bitcoin. For subsequent receipts, consider this options.
Based on your current situation, there is no way out than to comply with their requirement because they won't even allow you move your bitcoin out of the wallet but I hope the site you are using is a legit one as I have not heard about it before. Wishing you all the best there is more to learn.