What was recievd by you: offer letter or appointment letter?
Whats is the notice period inserted in it? Is it stated that T&C, service rules,HR pokicy as amended from time to time shall be applicable to you? Is it stated that service rules etc shall be placed at say; Employee's portal?
Is there any clause on notice pay in lieu of notice period in it?
Do you have copy of notice of resignation, proof of its disptach, proof of its delivery? Did you mention notice period/LWD in it?
Did you quote the clause numbers etc in it?
Has the HR or employer replied in writing to ask for serving 60 days notice period as per some revised policy?
Don't offer notice buy out option.
Let them supply you FnF statement showing payables by you and paybales by company and net payable.
Submit notice by letter addressed to good offices of appointing authority,MD thru Redg. Post.