There is a practical aid for companies and self-employed persons who wish to post employees abroad in Europe: the Service Compass.
The Service Compass website provides you with all the information (in German language) you need about posting employees. What is the legal framework? What are the tax regulations for invoicing? All these questions are answered here. You will also find contacts if you have further questions, for example about providing services abroad.
Source: IHK Swabia (German)