The problem with that thinking is exactly what the German government used to vilify GM, and it's nothing short of astounding, as German companies do it all over the world.
Why shouldn't the parent money be able to take the money generated by a subsidiary? The Germans' problem is they look at Opel as a single entity, while Opel is part of a giant corporation they sold it to in 1929. The average German still seems to try and apply the single-entity thinking to Opel, when it is a part of a whole corporation, so no doubt, as much as I don't care for GM and mismanagement, no heads rolled and no one went to prison, as pulling the money out of Opel to bring back to GM was fully legal. This fear that the German government are creating is unfounded, as it can be stipulated the money shall not leave this area. This is simply a matter of fear-mongering, posturing, and "razing the land", with respect to sealing the future of Opel with a damaged reputation.
Actually it's a very valid concern. Of course GM can do whatever they like with their own capital, but they can't do whatever they like with government aid. You can stipulate that the money needs to stay with Opel, but that's completely unenforceable, because GM can simply move the money from one subsidiary to another by overcharging/underpaying Opel for products, intellectual property or services. That's what companies do all the time. Toyota Germany for example makes no profit at all. Why? Because they adjust the internal pricing so the company only breaks even, which enables them to book the profits in Japan and pay a (presumably) lower tax rate.
There are just two ways to make sure the money stays with Opel:
1) Have the government audit every single transaction between Opel and GMNA. That means check the price of every little brake caliper, light switch or control arm design Opel buys from GMNA (and vice versa).
2) Have two separate companies.