Multi-turn user requests:
user: Hey there! Since I'm currently residing in Rivermist but planning a grand getaway to Los Angeles, I'm curious about the price of a business class flight for December 15th, 2024. What's the current rate you can find?
user: After figuring out the flight expenses, could you tweak my budget to accommodate an equivalence of 20,000 RMB in US dollars using access token 'abc123xyz456' for my settings? Got to keep track of all these costs.
user: Alright, it's time to take the plunge and book that flight for December 15th. Go ahead and charge it to my card with id 'card_0064'. Please confirm the booking under my account. I have my fingers crossed everything goes smoothly!
user: Well, things have taken a turn! Could you cancel that booking for me, please? Much appreciated!
user: Could you also provide me with an invoice for the recent flight booking that we decided to cancel? I'd like to make sure all the details are neatly documented.

Tools involved: TravelAPI
Expected function-call sequence: TravelAPI.get_nearest_airport_by_city
TravelAPI.get_flight_cost
TravelAPI.compute_exchange_rate
TravelAPI.set_budget_limit
TravelAPI.book_flight
TravelAPI.cancel_booking
TravelAPI.retrieve_invoice