There are laws in Germany that guarantee payment of appropriate wages and compliance with maximum working hours. These apply to all employers and must be obeyed by them.
Promised wages must be paid
Additionally, arrangements between employees and employers are usually recorded in an employment agreement. If disagreements arise, you can later prove what you were promised and what is expected of you.
If an employer does not comply with the obligations specified in the agreement – for example, does not pay the wages promised – you can go to a labour court. In many companies there are also employee organisations that will assist you.