Employer Demanding Salary Back

Company had asked me to go to USA on tourist visa. But it was illegal and trip did not happen Then company pressurized to do trip at my expense. They only paid me $3000 for residential expenses in cash advance but there were no guidlines or rules or agreements. Then I agreed to repay in installments. And resigned to do full and final settlement. They did not pay me salary for 1 month and notice period of 45 days which I had given with proper written notice. Now they have sent me notice to return all expenses and salary too and loss cuased to company amounting Rs. 20 Lacs. I do have all written proof of work I did in my tenure. They are asking for salary for last 6 Months till notice period and in email and notice said that they never accepted my notice period. But another dickhead director whom I was reporting had accepted and asked me to do handover which was all done legally. Also all company assets were returned. Company board members were all failure in all there business startyups But I made this one huge success. Due to their loss to expand it at my cost they are sending me these emails and notices. Also they threatned to defame my name in market. I want to now end this all legally what should be my next step.