Hello,
I don't know why you must make Verpflichtungserklärung with the Ausländerbehörde for a family reunion procedure. Generally a spouse who wishes a family reunion from abroad must provide proof of enough source of livelyhood for the whole family (in your case 2 persons) with his or her application in the embassy. As you earn more than enough and your contract will be valid more than 1,5 years, it mustn't be a problem at all. I´ve never heard about Verpflichtungserklärung for a national Visa for family reunion. Normally, you need such a document for inviting somebody to visit you or in some special cases. But Im not from Berlin or Brandenburg, perhaps they treat it in this way.
The problem with the visa application can be that the embassy has no right to issue it before they have asked the Ausländerbehörde in your place and got their approval (Zustimmung). If they don't get it, they would reject the visa application. In this case your husband can make a remonstration within 1 month and in case of rejection he can appeal to the court in Berlin. Naturally this all can take much time.
Theoretically you can apply to the administrative court in your place and ask them to oblige the Ausländerbehörde to allow you to make a Verpflichtungserklärung. But I've never heard about such appeal. And for that I would definitely consult a good migration lawyer.
Did you inform you about the conditions and necessary documents for family reunion on the web page of the responsible embassy? If yes, did you really find Verpflichtungserklärung in the list? It would be interesting from what country you make family reunion. Perhaps there is something special there.
I presume your marriage is legally acceptable for German authorities and your husband has A1 Certificate. These are mostly important problems for family reunion, together with having not enough money.