Any salary deduction without consent and valid reasons is a breach of employment contract. Definitely illegal.

Salary deduction can be made under extreme financial situation which the company may force to shutdown or conduct major downsizing. It has to be communicated to all employees, and the only way to save situation is salary deduction. Let the employees understand and request for their understanding and consent. If majority accept such decision, proceed with salary deduction.

I don't think you can do that.... Employee may file under constructive dismissal.