Help with a fake Unauthorized claim

Hi,
I have a weird problem here, and i was hoping i could get some advice from the paypal employees here.
I sold an intangible item to a guy on a forum. He tells me to send him an invoice for the payment, which i do. He cancels the invoice and sends me payment as a gift! This, after me telling him it isnt such a good idea as he is exploiting paypal rules. He says not to worry and he has done it a lot of times. I anyway go ahead with the transfer of item. A day after he gets the item he starts threatening me to refund him the payment, or he will file a claim against me. I try to reason with him but he then starts threatening me more. But when he tries to file a claim it backfires since he himself sent the payment as a gift. He then goes berserk and says he will file an unauthorized claim. And he did this today, with new stories of how he wasnt at his place and some hacker from poland got access and ran his e-Book business for him!!

My question here is, what can i do? I have all his emails that he sent me. I also have his messages from the forum where i met him, and he scammed so many people that his account even got closed there, so this is kind of a proof that he was untrue to many.

Please help me here, as i have a 100 percent feedback eBay account attached to my paypal and my paypal account is now negative by over $450..