Owing money I don't have help!!

I broke up with my ex he gave me 500 pound for rent and bills when I tried to pay him back he wouldn't take it this was in April. He got me a contract out in his name for my birthday but he started getting weird and if I didn't want to stay over he would say he was going to kill himself etc so I ended it and I offered to give him the phone back which he said he disnt want and only wanted me. I sold the phone because I didn't want it and forward 3 months and hes demanding this money off me. What do I do?