Declarations to Customs
When you enter into Korea possessing foreign currency exceeding the equivalent of USD 10,000, you shall declare the fact to the customs and acquire a Declaration of Foreign Exchange.
Exchange your foreign currency at KEB
When you exchange foreign currency less than the equivalent of USD 20,000, just visit a KEB branch with your passport without any kind of another document.
In the case of more than USD 20,000 or equivalent, visit a KEB branch with your passport, ?Declaration of Foreign Exchange and submit Application for Sale/Deposit in Accounts/Withdrawal.
(If you don't have a Declaration of Foreign Exchange, you should visit the Bank of Korea and acquire a Declaration Document of trading of the means of foreign payment.
After exchange, keep KEB's certificate of foreign exchange purchased or receipt of foreign exchange purchased.(They might come in handy when you exchange Korean won into a foreign currency.)