If I have to postpone my return to Germany, how can I get to the immigration office?

Generally speaking, in Germany, if the visa expires, as long as you have reasonable reasons, the immigration office will not give you any trouble.

For example, a student visa is no problem within one month after it expires.

For your business visa, as long as you have a business exit certificate and can explain why you didn't leave after the expiration. For example, work needs, air tickets and so on.

As long as you have the following three points, there is no problem:

Business certificate, that is, what are you doing in Germany?

2. Why does it expire? Be reasonable. As long as you don't want to leave with it.

3. Return ticket!

When the Immigration Bureau sees this, it will give you a "white note", usually a temporary "visa" to facilitate your work and customs clearance.

No problem.

As for the place of application, it is better to be the immigration office of the city where you first obtained the visa. No special department?

The sooner you do it, the better. The longer it takes, the immigration will think you did it on purpose.

It's the weekend, so it should be no problem for you to handle it on Monday, but there must be a "white note"