Company threatening to send a legal notice what can be done

This company was telling us they are Microsoft partners.After few months i realize that it was a scam . then i denied to work but they hold my 15 k out of my salary.They were not giving us appointment letter as well, If i joined the company in march ,they gave me appointment letter in October . In Dec i decided to leave that company anyhow with 2 more employees. it was 7th date so ,i went to office we all of them gave our resignation letter and the manager didn't signed it and told us to go home.And never contacted again.Now 2 days back on 12 Jan ,They sent me an email stating if i don't pay My one month salary to them .They will file an illegal case against me .They send me salary slip of december month as well even i never went their more than a month also didnt get any money w.r.t to that salary slip.They know i dont have bugs to pay them ,so they are indirectly forcing me to do scam. How can i do that .And why i should pay that fraud company as they aready hold my 22000