Ive heard about these German laws. I once had a client that needed an invoice for almost everything. Never did understand the laws up there but they sure as hell seem like sticklers for finances.
It's the exact same thing as an invoice except that the money flow is reversed. I've used this model when I had a bunch of different people doing promotion and I didn't want to wait for all of them to send me something on paper.
This way you can avoid the hassle of coaching Stanislav, the programmer from north Siberia, on local laws, and waiting for him to send you something back.