Hi everyone, 
I am working fo the past year with a 40 hour contract and with Blue Card. My employer reduced my work hours from 40 to 36 due to some reasons. This was not done contractually, I did not sign a new contract, just the Salary slip says Menge 0,9 and the Gross salary given is 90 percent of my gross salary as per contact.

This results in my salary being lesser than the Blue Card minimum requirements.

I am only aware of this now, when I got payslip for previous month.

So was my Blue Card invalid for last month?
So am I living illegally in the past month?
My employer never contacted the ABH and is not willing to contact the ABH as well.
What can be done in this scenario?
I am looking forward to some valuable answers please.

First of all, you signed a binding contract stating a particular salary and surely your weekly work hours. If you signed nothing else and your employer reduced your work hours "for some reasons", you first have to get in touch with him and clear this up.
You are living in a country where laws have to be followed to the dot and if your rights are violated, which is the case, you have to get to the bottom of it. Only after that can you start thinking about invalidity of blue card and whatnot.
Know your right and live a peaceful life.

D.