Can a non-EU citizen holding a Romanian student visa (Masters Degree), find a job in Germany and move there before finishing his study?
Not on the basis of the student visa. Work authorization for citizens of countries that are neither in the EU nor in the Schengen area is handled by national law. The only country that can issue a document permitting the student to work in Germany is Germany.
Furthermore, since (as noted in a comment) Romania is not part of the Schengen area, the Romanian student visa does not even allow the student to travel to Germany, much less work there.
Whether the student must wait for the end of the studies to apply for a German visa is up to Germany. If the student already has the qualifications for a German work visa without finishing the studies in Romania, it should be possible to apply for a German work visa, which, if granted, would allow the student to move to Germany before the end of the studies in Romania.
Yes, but it depends on how many hours is the job and what is the job related to...
I was a student too and i was able to have a side job (neben Job in German)
here some info:
Rules for students
Students from the European Union, Iceland, Liechtenstein, Norway and Switzerland enjoy unrestricted access to the German labour market and have practically the same rights as German students. However, if they work more than 20 hours per week they must pay certain insurance contributions (just like German students). For students from other countries, special legal regulations apply:
International students from other countries are allowed to work 120 full days or 240 half days per year. They are not allowed to be self-employed or work as freelancers.
Students who want to work more need permission from the Agentur für Arbeit (Federal Employment Agency) and the Ausländerbehörde (foreigners' office). Whether they are given permission depends on the situation on the labour market: the chances are better in regions with low unemployment.
An exception is working as an academic assistant. There is no limit to how many days academic assistants may work. They still have to inform the foreigners' office however. If you are uncertain what category a job falls into, you should seek advice from student services or the International Office.