Based on my experience, the user-triggered start-of-turn event happens after all of the game's built-in start-of-turn events. Same for the events that key off the end of a turn, start of a combat, creation of a unit, etc., so in the case you're asking about, the deal would expire and then your start-of-turn event could kick in. Assuming you used the save/load functionality to store variables across turns (like how the Building Resources mod works), it shouldn't be hard to have it trigger immediately after the expiration this way, before the user has a chance to notice.