Notice period

Hi everyone. I need some help. It would be great if you could help me with it. I was working with a company whose notice period was 30 days at the time of joining. Company revised the notice period from 30 days to 90 days. N after that within 20 days I resigned from the company and asked for release in 2 months. They released me early in 60 days. Then they sent a mail to me on my LWD, that confirm you will pay for the remaining notice period ( for relieving letter). As I didn't have any choice, I confirmed it and asked for the amount. But at that time they said we can't tell the amount right now. And now they has sent me the fnf settlement and buyout period is calculated on gross salary instead of basic salary and leave encashment is calculated on basic. I am not sure whether company even has any official document for this policy. And I have heard that notice period is calculated on basic salary As per my offer letter, my notice period is 30 days. And I have not confirmed and signed appointment letter of 90 days. So which notice period is applicable to me?