$300 for a cell phone? I thought they went for less than $100.
Anyway, unless you can prove you paid for the phone (and the amount) I don't see how you can get away without paying him. What is your mom's position on the matter?
I know this is a lot of money for a kid to come up with, but sometimes it is worth giving up the cash for a bit of peace. I can't say for sure if THIS is one of those times. That is for you to figure out.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?