There's a few seconds where he's unsure about Hibari, about whether he's kidding. Yamamoto just ends up laughing it off, because of course the other wouldn't waste his time biting if it didn't result in some kind of challenge or victory.

"Haha! Sure!" Yamamoto isn't welcoming it, but acting so flippant about it should help to convey that he doesn't want to become a chew-toy.