Notice period by company to recover amount in lieu of notice period.

Hello sir, I have worked in a Pvt company for only 5 months as a senior executive post in accounts department. They mentally pressed me due reason I resigned on 4 Feb 2017 and gave him a notice of 15 days. Now after multiple follows for my FNF they sent me the company notice stating that you have not paid the notice period recovery amount on the basis of gross salary and wanted me to pay 80000 thousand something. Even I am ready to pay the basis amount for non complete the notice period as no clause is mention in the offer letter to charge the gross amount. As per offer letter 90 days notice period is required, am I come in the labour category as my salary is 31500. They have not deduct PF to any employee. Can it be a ground to file a suite. Some one is saying they will file a civil suite against you. Pls suggest whether I have to pay the said amount. Or what to do I am in trouble. Can I send a legal notice to HR and manager?