I am working in a broking company. I joined this company on 3rd February 2014, and my NISM certificate was attached in the branch on that day. When I asked for an offer letter, they told me that it would come shortly. I received my offer letter on 11th March 2014, after 36 days of joining. In April, I received a salary from the date of the offer letter. However, they are not giving me my previous salary. What should I do?