I gave him the money through the bank. He took the money to the dark web space. There he gave the money to an exchange office. The exchange has transferred bitcoins to him on his user. A kind of payment gateway. My friend himself does not know how to transfer them.
So up to this point, the exchange has transferred 20BTC to your friend's account which he doesn't know how to use, correct? I still doubt that your friend got real BTC from that exchange and the exchange office he visited is a permanent office. As for the privacy contract between your friend and the exchange, we're talking 20BTC ($750k today) that you should have involved into the contract.
I don't know how close you are to your friends, but to me there is no reliable friend when it comes to big money.
By the way, why don't you also ask your friend to come here to explain the situation?