Is it permissible to deal with foreign currency exchange agencies at airports, and what is the ruling on working in them, given that the traveler pays in local currency at the bank in advance and then receives the foreign currency at the airport?

If the transaction between the client and the bank involves a promise of currency exchange, and the client deposits the local currency, and then the currency exchange contract is concluded upon presenting the receipt at the airport after completing travel procedures, then there is no harm in it. Currency exchange with a known existing deposit is permissible, as Ibn Qudamah mentioned in "Al-Mughni": "If a man has a dinar as a deposit with another man, and he exchanges currency with it, and its existence is known or presumed, then the exchange is valid." Accordingly, there is no issue in working with currency exchange agencies at the airport.