threat me through notice to return 6 months salary what to do

I've worked 6 months in a company named as flomic freight services in ahmedabad branch. during my joining i have signed appointment letter which mentioned if i leave company before 2yrs after my joining . i have to return 6 months salary to them as compensation. i was appointed as sales trainee.and i was on probation period. but due to some family issue i left that company with proper formal procedure like i return every material i used, I give proper resignation letter of 15 days before leaving, and i also didnt get my last 45days salary. now they are sending me notice to return 6 months salary to them. what should i do.