Окей, я нашёл вот:
What does the 30% ruling actually mean in financial terms? From a tax perspective, the salary agreed upon between the employee and employer will be reduced by 30%. In return, the employee should receive a 30% allowance as reimbursement for expenses. This is the most common way it is applied as it does not influence the salary burden for the employer. However, the employer is not obliged to pass on the advantage of the ruling to the employee. In practice the employer can partially or fully take the benefit.