Ex employer demanding compensation (after 1 year) for my voluntary abandonment of service

I joined a corporation last year and had to quit on the first day due to certain medical conditions and informed about the same to the concerned HR the very same day. But now after almost a year, they are demanding me to pay them compensation for leaving without serving the notice period even though I just worked for one day and I was on probation at the time.
At the time of my voluntary abandonment of service, I asked the HR if anything had to be done on my part and he informed me of no such thing.
What should I do?

Have they given anything in writing?
Pl share correspondence duly masking names etc
They have no chance of winning any legal battle on this ground

Yes, they posted me a letter detailing the amount of compensation due.
Also, they opened a bank account on my name even after I told them that I won't be joining the company on the second day only and they credited certain an amount. But I have no authority over this account nor do I have any papers relating to it.
It is a leading private bank.

Did you fill up any bank a/c forms on first day?
This leading bank would not have opened a/c of an employee without photo,signatures and other proof.

It was part of the first day formalities. Opening the salary account, but I informed about that to the concerned HR at the time of leaving. And moreover, I received the account opening message on my phone number months later.
Plus, I have not used this bank account nor did I receive any passbook or any such papers relating to that account.