Freelance employer taking advantage of agreement what to do

I used to provide freelance IT services. From a freelancer site, I met a person and agreed to provide him some IT services for some particular amount(30000 INR). I signed an agreement document, and provided him my PAN card as prrof. When the work was in progress, I got paid by the employer according to the work completion which is in total 15000 INR. Employer kept asking and including new things, so I stopped working. Now he is asking me to return all the money, and threatening me that he'll file an FIR against me or lodge a case. We both live in different city, I want to know if this is legal? Or is it enough to file an FIR. He paid me after receiving the work, and still haven't paid the full amount for which we have agreed upon. What should I do?