The main rule is that you are subject to Dutch wage tax when you start employment in the Netherlands. The 183 day rule in the tax treaty provides a possibility to prevent becoming subject to Dutch wage tax, but if an invoice is send to the Dutch client for the work done, then the 183 day rule does already not apply.
There is a possibility if the employee is send to the Netherlands within the group of companies, the employee has specific expertise that makes him subject the 30% ruling, then he is regarded not having become subject to the Dutch wage tax if his stay in the Netherlands is less than 60 days.
The 60 days is counted per 12 months period, illness days are regarded working days. Weekend days spend abroad or any other day spend abroad during the 12 month period is not regarded a working day in the Netherlands.
