Understanding Salary Deductions and Legal Recourse
It is not fair, but it will depend on the conditions if any. As per the law, an employer is bound to pay only the statutory wages fixed by the government, and it is called minimum wages. When it is more than minimum wages, the employer can put some conditions. It may be an implied or express condition that when a salary is revised based on the performance of the employee, it is expected that he would remain with the employer at least until the next evaluation cycle.
A salary revision is also made to make the employee take on more functions. The employer would obviously expect something extra. If he leaves after collecting a few months' revised salary, the employer can revise the order of salary revision citing the reason that in the revised salary bracket, you are not competent. But there should be communication to this effect before the deduction is made. The employee should also be given an opportunity to be heard on the charge, i.e., not discharging the functions in the revised salary grade. Failure to give that opportunity is an error on the part of the employer.
Challenging Salary Deductions
But how can you challenge it? Since your salary is Rs 90 thousand, you will not come under the scope of the Payment of Wages Act, and hence you cannot get the protection of that Act. Then you should choose a violation of the contract between you and the employer and file a civil suit. How long will it take to get a verdict? Decades, I must say. Therefore, if you can make any amicable settlement, you can initiate that. But I don't understand how it becomes just Rs 20 thousand?