I didn't send him an invoice. He's not smart enough to elaborate properly. I paid him 50usd by sending it in paypal (using the option: paying for services and goods). I didn't knew that this was the wrong option. So, the money is held by Paypal until he gives Paypal an package ID (this is the buyer protection thing).
Now the money has been withdrawn from my bankaccount. But I certainly did not receive the 40m I paid for yet ;)