Au Finance bank - Wrong last working day mentioned in relieving letter by Employer

Dear team
I resigned on *th July and my notice period was * months, I served till **th September and as per * months notice period my last working day was *nd October.... But since I had to join other organization on **th September I sent multiple mails for relieving me just * days earlier from my last working day.
As per HR policy 'Either you need to serve notice period or need to compensate gross salary against shortfall of notice period' hence as per labour law Employer can't force me to serve notice period as I was ready to bear recovery against shortfall of notice period

When I didn't get any confirmation I myself sent declaration mail for my last working day as **th September and also wrote that I am ready to compensate against my remaining notice period as per HR policy. I joined other organization on **th September but my previous organization intentionally mentioned last working day as *nd October

After getting relieving also I sent multiple emails for correction of date but didn't get any help

Please suggest what steps can be taken against my previous employer for this unethical behavior...