Could trade at the deadline (the waiver deadline, so August 31) for an AL starting pitcher who ends up AL Cy Young (and had a bonus clause). Who's on the hook for that bonus?
Pretty sure when a player gets moved, the teams either agree to a complete 'Here you go...' in which the new team owes the full amount and any bonus obligations, or the new team just picks up the league minimum and the old team is on the hook. The second situation is usually only when it's a player that wasn't going to be playing in the majors much of any other way, which wouldn't be the case for a Cy Young candidate.
Therefore, I'd expect the new team to be on the hook.