As a little bit of background, I have been working in Germany for about a year now. It is a startup company with less than 40 people.
Last December I received €1.7k more than my usual salary. I reported this to my immediate manager (who is also one of the partners and top management). He said sometimes it happens at the end of the year, and it is ok. I waited for a couple weeks and there were no follow ups. So I used the money.
Today, my manager told me that it was actually a mistake at somewhere and I should pay the money back.
What options do I have? Is it possible to reject such a demand?
There is a law for pretty much everything in Germany. Is there a law or at least a widely accepted rule for such a situation?
To be clear, my manager is a really cool guy and he was also very uncomfortable while giving me these news. So I really don't want a problem with him.