I'm not sure whether this is helpful, since my situation was different (I say that not knowing much at all about your situation) and it was several years ago, but:
My first Aufentshaltserlaubnis had a stamp on it saying I was not allowed to work "selbständig" (I had a part-time job in an office at the time). When I started working as a freelance translator, I wrote a letter explaining that I would like to get an exception to the rule in that stamp, just for the job of "Freelance translator," and took the letter to the Ausländerbehörde. (I also did some teaching, but didn't mention that, for no particular reason). The exception was approved, and duly scribbled into my passport.
(After I'd been here 8 years, I got an unbefristete Aufenthaltserlaubnis, today called a "Niederlassungserlaubnis, and I no longer needed any kind of work permit.)
One important note for anyone setting up as a freelance translator:
When I did that, I got a letter from the Gewerbeamt telling me I had to register a Gewerbe – and that's not true (fortunately, I'd been warned.)
"Bei der Tätigkeit eines freiberuflichen Übersetzers handelt es sich um eine Tätigkeit, die dem gewerblichen Bereich nicht zuzuordnen ist. Ich bitte hierzu um Berücksichtigung von Paragraph 18, Absatz 1 des Einkommensteuergesetztes (EStG)."
(I got a letter from some eager beaver at the tax office recently telling me I had to provide proof of a Gewerbe, and proof that I'd been paying Gewerbesteuer for at least the past 3 years, so I sent them a letter with the above statement.)
PS: Apparently Ausländeramts can differ a lot from one city to the next, as far as how helpful they are. It might be worth your while to look for a lawyer who specializes in Ausländerrecht - some will give you an initial consultation for 50 euros and tell you exactly what the law is with regard to your own situation.