Can a company hold my original marks sheet legal if I resign?

 I have agreed verbally to work 6 months minimum for a pvt mnc company. On the joining date I was told there are few more employees joining and my shift timings are from 6am to 1 pm. From the beginning my boss was always negative and continuously harassing and degrading even for petty issues. I had plans to leave and informed them on WhatsApp. They tried to convince me before I thought anything my mom fell sick and I had to attend her. Now they are asking me to pay back the salary they paid as it is company's law and also written in MOU in which I signed. They with held my 10th original marks certificate. I'm a single employee in a remote area neither they gave a payslip nor they gave me any identity card. I joined on Jan 16 2017 and quit on March 9 2017. They didn't pay my training fee. Kindly help me. They claim it is legal to hold my original marks sheet.