There's a short story I've been trying to find. I read it in an anthology of time travel stories and I can't think of the title of either the story or the anthology for the life of me. The summary is this: A guy travels into the future, but he has a limited amount of time (hehe, irony) so he desperately asks a citizen of this future time for proof that he can take back with him. The person he asks tells him to take off all of his clothes in exchange for a pen. Then the person winks out of existence, and we learn that this happens quite often. The people in the future are always playing this kind of prank on these people because when the time traveler returns to his own time, he has no memory of his trip to the future, no clothes and all he has is a pen, which doesn't prove anything. Does this ring a bell to anyone? Has anyone else read the story and do they know where I can find it again?