If there is no service period, only the unit is notified to resign three days in advance during the probation period, and the penalty clause is invalid.
Labor Contract Law
Article 22 Where an employing unit provides special training fees and professional technical training for laborers, it may conclude an agreement with the laborers to stipulate the service period.
If the laborer violates the service period agreement, he shall pay liquidated damages to the employer in accordance with the agreement. The amount of liquidated damages shall not exceed the training expenses provided by the employer. The liquidated damages that the employer requires the laborer to pay shall not exceed the training expenses that should be shared for the unfinished part of the service period.
Article 25 Except under the circumstances stipulated in Articles 22 and 23 of this Law, the employing unit shall not agree with the laborer that the laborer shall bear the liquidated damages.
Article 37 A laborer may terminate the labor contract by giving a written notice to the employing unit 30 days in advance. The employee may terminate the labor contract by notifying the employer three days in advance during the probation period.