Salary is not Paid

As per appointment letter its 2 months of notice period. Employer and employee mutually agreed for early relieving verbally on 14th april. Employee sent resignation on the mail on 14th april and worked for 23 days till 6th may, which was agreed by employer on Whatsapp Chat. He also committed on chat that 6th May will be the last working day and pending dues will be cleared. Now employer is not paying salary for 36 days in total (since of 1st April to 6th may). Director of the company also got one letter signed which was for early relieving, it was mentioned that u can be relieved early and few basic terms were mentioned in that letter like you will be available till handover, you will not spread negativity, etc. and now suddenly he is sending a mail that we are getting another document made from a lawyer, you have to sign that, after that full n final will happen, else we will take legal action against you". I want to ask that When company itself has given me a signed letter on a letterhead for early relieving whose terms i have met and they don't have any proof that i didn't follow those terms, so now how can company send a legal notice for not serving the notice period. I already have that document which they had signed for relieving me early.