body: Is it legally possible for a European (non-German) PhD student/research assistant at a German university (who gets paid according to 75% of the TV-L 13 wage) perform side jobs for international clients (for example, through a platform like UpWork)? 
  
  
  If yes, what are the rules regarding tax declarations, release of receipts, VAT, and so on?
  
  
  
title: Tax law and freelancing as a side-job in Germany