Got legal notice from employer

I had joined a CA fim in the month of june for learning purpose and clearly said the employer at the time of interview the purpose of my job. After the period of one and half month he deployed me in a government organization namely SFAC as he has some contract there. I had worked in that org. for sometime but there is no learning portion in the job description. Some repetitive work is provided. I inform this to my employer that there is no learning. But for his contract he asked me to continue there. So i decided to change my job and informed my employer. He asked me to serve the notice period of one month. I served that notice period and informed him formally over email of my resignation. But even after that he haven't given my salary for the month of September and sent me a legal notice that i will not demand for salary. 1. My primary reason of leaving the job is work... I'm not getting anything to learn. 2. His behavior is not good at job or can say...is insulting. 3. He even never pays salary on time. What should I do now ?