Salary being with held by the company no written proof of communicatio

I submitted my resignation letter on the 17th of April,2017 and was accepted on the same day. Also on a telephonic conversation that I had with my HOD, I was given two options : 1- I could be relieved immediately and would have to pay the company a months salary and 2- I could work for the rest of the month and I would be paid my salary for the month. I completed the month there and handed over all the assets. I tried getting in touch with my HOD from the 2nd week of may but he'd always be busy. Finally when I got in touch with him on the 15th of may, I was informed that the company had taken a decision to with hold my salary due to various reasons which would be communicated to me over my personal mail id. Now, I know that i do not have a written proof of the communication we had but also there was no mention that my salary would be with held by the company in the acknowledgement of my resignation letter. Please advise how I could get back at them .