Nice to hear from you again!
I think it was a mistake to withdraw the asylum application because your brother didn't get his residence permit yet. As long as he was still in the asylum procedure he had an Aufenthaltsgestattung which made his stay legal.
Now the situation is the following: It's true that he can get deported even though he is married to a German. At the moment he neither has a residence permit nor is he an asylum seeker with Aufenthaltsgestattung. So it's lawful for Ausländerbehörde to just give him Duldung and to ask him to leave the country after it expires (respectively to deport him if they get his passport).
The dilemma is that he most likely has to submit his passport at Ausländerbehörde when he wants to apply for the residence permit anyway. So Ausländerbehörde could still deport him at that point. And apart from that it might happen that he has to go back to his home country in order to come back to Germany with a proper visa. Then he will be able to get a residence permit. Please check out the following link for further information:
May I ask what your brother's lawyer recommended to do? He probably has a much better overview of the situation and can give more precise advice.