Offer letter is issued after selection in interview and candidate is asked to accept it! Do you have a copy of it?
If yes is any retention clause stated in it?
Is retention clause stated in appointment letter?
You have posted that retention clause is not stated in Service manual too!
Did you record this call of HR during which the retention clause was highlighted?
Record the call again.
Submit a gentle representation addressed to god offices of appointing authority, MD and narrate all representations made so far ((with dates, phone numbers, names and designation of company’s employees contacted, brief minutes of discussion), explain that you have served notice period and have handed over the charge in office and demand to supply you the acceptance of resignation, service certificate, relieving letter, FNF statement, FNF dues by bank DD only (thru redg. Post),Form 16 as per correct FNF statement, PF number (if applicable) etc……………………by redg. Post only. Clarify that Mr/Ms................................HR on dated.............................has demanded some retention charge without any documentary evidence,which is not applicable to you as per offer letter, appointment letter, service manual issued to you. Hence intervention of oood offices is being sought to supply all of above say within next 7 days.........................or as suitabel to you.
You mention that postage prepaid self addressed envelope is attached!
If good offices also do not resolve then you may approach your lawyer!
What is this establishment: Commercial, industrial, small enterprise/
How many employees are employed in it?
You are located in which state?
Redg. Office the company is located in which state?
What is your designation and nature of duties?