Theory
The idea is that an individual's income stays the same. There are some steps on how to determine the tax: Calculate the amount of money paid on taxes in an individual's home country. This sum of money is the hypothetical tax liability. Reduce the pay of the individual by his/her tax liability. Add any allowance that is necessary to be paid while he/she is abroad as a result of an assignment. This is his/her net assignment pay.Examples of complications
Although it may appear easy to calculate net pay, there are some other difficulties influencing your net pay. For example, if a company car in the home country is used, should some subsidies be used by someone abroad and unable to use it? Or how is the partner's income treated? For whom is the split-year tax system beneficial? What if an individual leaves the company? Does your tax equalization policy discourage individuals from acquiring property abroad?Practice
In practice, it is usually about agreement of both sides. Both parties should know on which basis is the tax calculated. After this is calculated, the amount is deducted from an individual's net pay on a regular basis throughout his/her assignment abroad. It is common, that the company deducts a hypothetical liability at the beginning of the year and then undertakes tax reconciliation at the end of the year.References