Defamation charges.

My client who happens to be my school friend, approached me to design the interiors of his resturant. But after i handed him all the drawings and related things he did not talk about my fee. When i brought it up he said he had no idea and asked me to send a bill (he works for his father's developing firm and he is well aware of the rates and fees). I sent him the bill which he refused to pay so we negotiated and reached to a settlement, with me accepting 40% of the quoted amount, since I have just started with my career and he being a friend. Later he kept on saying he'll pay me tomorrow, in a week, in the evening etc. Two months passed and he didn't pay up. I also had a friend working on it for me. He produced the graphics for the project since i couldn't do it myself due to a situation. After two months of false promised he then paid me 5% of the quoted amount. I wrote about my and in general an artists plight about these kind of issues and also tagged him on facebook. He now says he'll file a defamation case against me. I have all the emails and text message exchanges with me.