McDaniel surmised that Sir Whines-A-Lot had been luring passersby to the glass while his coworkers weren't around, prompting them to try to interact with him through the slot. In that scenario, a bill would make for a convenient cat toy.
“I’m guessing the first people didn’t intend to lose their dollar,” McDaniel said. “They probably put it in to mess with him, and he snatched it. When you hold money there, he’ll bat at it playfully at first, like, 'Oh, that's kinda fun.' But then all of a sudden he'll bite it and rip it from your hand. He does it so fast, you don't even realize what's happened.”
Here's video of Sir Whines-A-Lot in action.