How do the Japanese dollars serve as a symbol of the King's influence in the camp?

The Japanese Dollar – sometimes referred to as “counterfeit dollars” – is the currency used on the islands the Japanese invade in WWII, including Singapore Island, and is used in Changi Prison by the prisoners and Korean guards alike. It is also the currency used in the village where the King goes to make the deal for the diamond. After the King completes that deal he has thousands of Japanese Dollars and believes that he is set for months. A few days later, however, the war ends when Japan surrenders. The currency becomes worthless, which ruins the King.