Actually, if you are on a student visa you can apply for a working permit as a self-employed. You have to proof though, that this is not affecting your studies. So, if you work as a software developer, it's more likely to be granted if you are studying IT rather than philosophy, for instance. Having a project that is by definition limited in time and paid hourly is more likely to be accepted than fixed-price projects that might require you to do a few night shifts to finish the project.
The easiest solution for you would be to find a small company that is willing to employ you. In Germany we can't pay employees an hourly salary. Even if a company does not have work for you, they have to pay you. Companies with more than 10 employees are also subject to extra regulation and can't lay you off. So what you are looking for is a small software company with no more than 9 employees. If you have a few projects that you could transfer to the company so that your salary is paid for, you increase your chances of being accepted.
Keep in mind, though, that costs for employers are high, around 100% of your net income. If you get 50% of the project cost that's actually just breaking even for your employer.
