Comment by LinaLauneBaer

You have every right to profit according to German law. You have to be compensated appropriately for your employer to be allowed to use what you hold the copyright for. Usually this is ensured by your monthly salary. However, if what you did and hold the copyright for becomes a surprise success you have to be compensated appropriately in addition to your salary. In practical terms, if your code makes a billion bucks and you only get a regular average salary you could sue the employer and would probably win.