I would ask in the university if they allow you to continue your master studies as a working person (with a residence permit for work). There are special rules for people who make their studies parallel to normal work (job-accompanying studies). Some german colleges offer special study courses for this. If your study course doesn' allow such an option and the university knows about your work and let you study, they can have problems. Normally a student cant't work more than 20 hours a week. With a residence permit as a student it is not possible to work 80%, it is against the law.