Salary holding by employer

Hello i joined one engineering company in Mumbai as a Purchase & Stores Executive after getting their offer. I served there for about 20 days & i came to know about the company's financial background which was very poor as salaries would go to 15-20 days delayed from the date of Salary. I got a better offer from somewhere else so i decided to leave & put my resignation to concerned authority after discussing with HR & Reporting Manager. i did not hide anything from them & left. Since now i haven't received those 20 days salary for working there. i am following up with mails & phones but now they are asking me to serve 15 days notice period which was never mentioned neither on offer letter or orally from HR. They even did not gave me appointment letter while joining but still ask me to serve notice period. How can i get my salary for working those period in the company? please guide.