Even if you work with Dutch employees and a Dutch employment contract, as soon as there is cross-border work in Germany you must also comply with a number of German labor law provisions.
For example, you must pay the German statutory minimum wage. Does your staff fall within the scope of a German collective labor agreement? Then you will have to pay the (higher) minimum wage from this collective labor agreement. In addition, your staff will have a number of other rights under German law, such as the right to continued payment of wages during a holiday period. You must also take into account German working and rest times, occupational health and safety legislation and rules for equal treatment.
To demonstrate that you meet these requirements, you must document a number of things (in German!) and keep these documents at a German address. This includes the employment contract, a recent pay slip, proof of payment of wages, a time sheet and a location notification.