Company is keep delaying my last month salary and full & final

I worked for an Noida based well known company, I left the company after serving full notice period. Before that company paid me with some deputed amount as I was at client site and they formally sent an email if you leave company before 6 month they will recover it at the time of termination but I left it in 4.5 months. When I resigned and went for clearance, they even didn't discussed about any deputation amount. Simply they provided clearance with "No DUES" and experience letter, I have copy of clearance latter. I thought, they wived off the amount for good faith as I was working from past ~2.5 year. But at time of full and final statement, they are denying to pay any amount even asking for money for full and final statement. When I discussed all these things, they accepted mistake is from their side as they haven't notified at the time of clearance/termination. Still they are not responding my emails. What should I do now, should I wait or send them legal notice?