Why did the dealer rule his hand dead? In this situation, the default ruling would be that the cards can be retrieved if they are clearly identifiable and retrievable. It is questionable whether you can even "fold" at a showdown, and even then, unless there is a house rule to the contrary, merely throwing your cards towards the muck is not in itself a binding fold. Now if the dealer had buried those cards deep into the muck such that they were no longer clearly identifiable and retrievable, that would be a different story.