Company harassment

My new company bought me out from previous employer by paying 75k to my account and asked me to pay it to my former employer and asked me not to serve the notice period in previous organisation. Once I joined the new employer, there was nothing mentioned that I have to serve the company for a specific time period. I had to leave the company since I was sick and being harassed for the same because I asked to work from home for four days. I decided to put my resignation. I served there for three months. Along with this they with held my one month salary before I had put down my papers. I was sent a letter that they want me to pay the notice period amount (one month salary during probation) and the buyout amount which was paid by them in the beginning. Additional to this, in no documents that I signed on it was mentioned that this amount will be reimbursed from me under any condition/circumstances. They are giving the reason of behavioural and performance issues and that is why they want the amount back but I put down my papers and they duly accepted the resignation.