Why mark your own reply as the solution, especially given that it has absolutely no relevance to the current ebay payment system.

Never heard of managed payments, you should not be selling.

Does your ebay page have a payments tab, that is a good place to look, and if so then you have signed up to managed payments.

Just to add, if the buyer has opened an official case, you have 72hrs to resolve the matter, or ebay will automatically refund the buyer.