According to the Employment Ordinance, it's prohibited from making such deductions unless your ex-employer has obtained the written approval from the Commissioner for Labour, or under the following circumstance:
Deductions for damage to or loss of the employer’s goods, equipment or property by the employee’s neglect or default. In any one case, the sum to be deducted cannot exceed $300. The total of such deductions must not exceed one quarter of the wages payable to the employee in that wage period.

Employers who make illegal deductions from the wages of employees are liable to prosecution and, upon conviction, to a fine of $100,000 and to imprisonment for one year.

Despite the above mentioned restrictions on salary deductions under the Employment Ordinance, your employers may try to claim damages/compensation($8000) from you through civil proceedings.

If you have any further queries, please do not hesitate to contact our staff at 2779 7922.

For details of the conciliation and consultation services to employers and employees on matters relating to conditions of employment and their rights and obligations under the Employment Ordinance, please approach the nearby offices of the Labour Relations Division during service hours.
For more details, please refer https://www.labour.gov.hk/eng/tele/lr1.htm