Resigned from a company. Co. not registered. No employment contract.

Hello ! I had given the Resignation from the Company , Company is Not registered and even I have not signed any employement agrrement related document, but suddenly due to bad health I resigned from the company and I had taken last month salary as I worked for the last month, but they are demanding the last month salary back but I don't want to give salry back as it is my earning for the work , does they can make any legal action over me . as they have given me soft copy of the agreement but I Havent provided any signed hard copy to them . Help me in this situation .