According to Article 40 of the Labor Contract Law of People's Republic of China (PRC), the employer shall provide necessary working conditions for the workers and sign a written labor contract with them. A labor contract is a text that clarifies the labor relationship between the employer and the employee, and clarifies the rights and obligations of both parties. Generally speaking, a supplementary agreement is to modify or supplement some specific matters of the employment relationship on the basis of the original labor contract.

If the supplementary agreement stipulates that after-service salary is deducted as training expenses, whether it is legal or not needs specific analysis. First of all, if the employer informs the new employees that they need to receive training for a certain period of time when they hire them, and both parties agree in writing that they need to bear the training expenses after leaving the company, it is legal to fulfill the agreement. However, if the employer unilaterally makes a supplementary agreement and forces employees to sign it, it may be illegal.

In addition, according to Article 42 of the Labor Contract Law of People's Republic of China (PRC), the employer may agree with the employee on a probation period, but the longest probation period shall not exceed six months. During the probation period, either party has the right to terminate the labor contract without paying economic compensation. Therefore, if the supplementary agreement involves the deduction of training fees outside the probation period, it will be determined whether it is legal after analyzing the specific situation.