Salary Deduction For Not Serving The Full Notice Period

I left my previous company 3-4 months ago and my company notice period was 2 months. When I resigned from my company then I asked them if they can release me in 1 months rather than 2 months and they said OK. But when they prepare my full and final payment sheet they deducted 1 month salary from my full and final payment saying I have not served 2 months notice period that's why they deducted this amount. (I worked for 50 days and they are only paying for 20 days. They are deducting 30 days salary as I asked them if they can release me 1 month early). Is it legal. if not then what kind of steps we can take against them. Thanks,