i got scammed plz help!!!!!

I was in spawn when someone challenged me to pvp. I said sure. He told me to put my chainmail armor on and fight him. I said no because I don't want to lose it. He said don't worry we'll do give backs. I said "u promise". and he said yes and he won and wouldn't give it back. then he died and now ndm57dcraft has it and wont give it back. I have screenshot evidence. I dont know how to add images. so plz let me know how to do it so I can show u the evidence