CMS, you are suggesting you are supplying consultancy services to the German.
No, he says that he has some consulting work to do for a German company.
why are you not issuing a invoice from the Polish to the German as is standard for professional services.
One would imagine that the German company want him to be employed by them. Perhaps their insurance requires that. Perhaps he doesn't want to take legal responsibility for his work. Whatever the reason, the person doing the paying gets to chose how they pay; if they want him on their books in order to do the work, he is on their books or he doesn't do the work.
Consultants in my world are self employed,
The thing about consultants is that some people who claim to be consultants are actually employed by nobody. I remember an Indian idiot in Warsaw who claimed to be doing consulting work for one of Poland's largest operators of convenience stores but who in reality didn't even know the names of the two brands of convenience stores that that company has. Any moron can claim to be a consultant, and some morons most certainly do. People who actually are consultants know that they get paid in whatever way the client wants them to be paid and that if they don't like the way the client intends to pay them their only choice is to not do the work.