The employer can't withhold the money.

Labor Contract Law

Article 9 The employing unit shall not detain the employee's resident identity card and other documents, and shall not require the employee to provide guarantee or collect property from the employee in other names.

Article 84 If an employing unit violates the provisions of this Law and distrains the resident identity cards and other certificates of laborers, the labor administrative department shall order it to be returned to the laborers within a time limit and punish it in accordance with relevant laws and regulations.

If an employing unit, in violation of the provisions of this Law, collects property from laborers by way of guarantee or other names, the labor administrative department shall order it to be returned to the laborers within a time limit and impose a fine of not less than 500 yuan but not more than 2,000 yuan per person; If it causes damage to workers, it shall be liable for compensation.