MEANING OF ”EMPLOYER” IN INTERNATIONAL TAX
The “employer” in international tax situations is the “actual employer”; the person or body who carries the relative risks and responsibilities to the work concerned. So, the formal contract is not decisive when determining on who the employer is. The Dutch court decisions seem to indicate that there must be a direct link to the work done and the cost borne (through a cross charge of salary costs) by the employer.
Other items when working internationally
Please note that all of the above considerations are regarding income taxes (and payroll tax) only. Your social security position is governed by a different set of tax treaties, or by the EU regulations with regard to social security. It could be that a social security obligation is triggered, where still there is no income tax due. Your work permit is a different topic altogether. In case you need one (being a non-EU citizen), there is usually only an exemption for short business trips.
Referring to the text above regarding the 183-day rule and other rules triggering the right to levy tax by the work country, it is sometimes beneficial to create taxation in two or more countries to lower the total income tax paid on gross salary.
Split payroll will often only be accepted in case of a genuine split payroll situation, which entails that the employee must physically work a few days or weeks out of the month in both countries, and that, economically, he is working for entities in both countries.
A salary split may lead to a lower overall income tax paid if the country of residence uses an exemption method rather than a credit (of foreign tax paid) method.