I am working in a broking company.i've joined this company on 03feb2014 and my NISM certificate were attach in branch on that day. when i asked for a offer letter, then they told me that that'll come shortly. i got my offer letter on 11 march2014 after36 days of joining. and in april i got a salary from the date of offer letter. now they are not giving my previous salary. what should I do.