Legal Notice After No Dues And Relieving Letter

Sir,
i was a trainee in a company for 6 months. I signed an agreement of 6 months with company but i quit before 6 months. I have not given any resignation letter but the company HR let me leave the company and I was given no dues certificate and relieving letter from the company.
Now after one month the company calls me and asking for the training cost in the agreement.
my question is that... weather they can take any legal action against me after giving me NO DUES and RELIEVING letter.
please help me in this situation.....
my friends are telling me that no action can be taken after no dues and relieving letter from company.....
and you have not given resignation so company has given you no dues and relieving letter on their behalf.....
please tell me if the company can take any legal action on me after no dues and relieving letter ?????
23rd May 2012 From India , Jalandhar

Dear Hi, Let me know about the agreement which you have signed or if you have copy then upload. then only i can suggest to you. regards, Ashutosh verma Advocate
25th July 2012 From India, Delhi

dear mr jaggaji,
It all depends on what service agreement you have signed with the co. If the co. has entered into a damages clause for non completion of the agreed period, they can file suit in court of law.
28th July 2012 From India , Pune