Thank you for this important hint! That was it! I didn't noticed that I made changes on the files for frontend, not backend.
Now I'm only wondering why I can't see any currency of some houses after saving them again (with the new settings). But I've found an interesting error as I changed my currency settings:
Warning: Invalid argument supplied for foreach() in
/html/administrator/components/com_realestatemanager/realestatemanager.php on line
4253
EDIT: I've found a hint inside this file about Paypal. But I don't use this feature.
I'm only showing houses with this component.
EDIT 2: If I want to save some houses, I get a red input field for garages. Our customers uses "yes" or "no" as values. So I've tried to remove the is valid number code in realestatemanager.html.php. But I still get the error of the red input field. I have no ideas now what I'm doing wrong there.