Odd.
The problem is, you don’t know if you’re dealing with an idiot who doesn’t know how to eBay and has opened a case when they didn’t need to, or someone who is intentionally being malicious.
You’re best bet is to wait it out till the weekend. Hopefully case gets closed. If not, then he will either escalate it, or it will elapse. The latter gets your money unfrozen, and the former has eBay deciding what’s what. Seeing as you have tracked delivery you should be ok.
Some more info here:
http://pages.ebay.co.uk/help/seller-protection-faq.html#What if the buyer received the item but still files an ‘item not recieved’ case?
eBay really is a pain in the balls these days.
Personally, I would have been tempted to refund him his money and keep the frame once he opened a case.