Intrest On Due Payment

I have payment due since 9 months on Dept. of Post for some work done. One senior officer granted verbal work order by approving quotation in writing (copy with me) and instructing local suptdt. level officer to issue work order but he didn't issued. I completed the work and suddenly senior officer got transfered now local suptdt. is harassing me for payment since 9 months. I have installation certificate, copy of quotation approved for work and pictures of work done. How can I force to give payment legally? I want penalty for harassments and delay.