I have this freelance job by a company which is not registered in Germany and I am going to Germany on study visa. Study visa allows me to work 20 hours/week. My company will wire money to my home country and not to Germany. Can I work on this job while I don't get paid in Germany and can I work more than 20 hours a week? To be precise, I am working in Germany on Study visa while getting paid in my home country.
If you are not being paid in Germany, not paying German taxes, and it is not affecting your school work, then I think you can. So long as you keep your grades up the German government will not know that you work more than 20 hours a week as a freelancer. If they ask, I would state that you work for the company, but stretch the truth that you work no more than 20 hours a week.
It is important to know though, that if they catch you lying on this paperwork, that you can get in trouble. It varies on how much trouble, but it is a possibility. (In the US not abiding by your agreement while on Visa status can get your visa revoked) however as a freelancer it's a lot harder for them to track down exactly how much you've worked.
Hope this helps.