Punjab state cooperative bank 3 months salary demand - appointment letter has no specific resignation terms

Query: I had been working in punjab state cooperative bank up-to 21october 2017. During my service period I applied for the post of inspector in cooperative department of punjab govt through proper channel. I got selection in cooperative department of punjab govt.. I joined the said post by tendering technical resignation to the bank as they had permitted me already. But now they have issued me a letter to submit three months salary to the bank as I have not served notice.. My appointment letter does not specifies about resignation terms.. What kind of legal remedy I can avail?? Please inform any law, circular, court order in this regard.
Thanks

Dear Angad,
As you have joined the government service, the employment certificate from your immediately served private company does not matter. Secondly, you have written that in the appointment letter, the provision of notice period is not mentioned. Therefore, you may give a reply to your ex-company that notice period was not served as there was no such provision in the appointment letter. Write further that no correspondence will be entertained on the subject.
Thanks,
Dinesh Divekar