The amount of money from South Korean defense contributions that US forces in Korea have not used but are keeping in reserve was 738 billion won (US$660.37 million) as of April, the Hankyoreh has confirmed.
If this amount is added to the other 531.7 billion won that South Korea has been obligated to pay but was not used by the US military, a total of 1.27 trillion won (US$1.14 billion) has been contributed to defense cost sharing but not used. On July 23, the Ministry of Defense submitted annual data on the accumulated amount of unused military funds for construction in the possession of US forces to United Progressive Party lawmaker Lee Seok-gi. According to this data, the money that the US military has received in cash from the South Korean government and is holding on to was found to be 738 billion won as of April. This figure includes 11.52 billion won from before 2005. The US army even still has 1.02 billion won that it received from the South Korean government in 2002, more than 10 years ago.
In the past, the South Korean government had indicated that it would not make an issue of this money, which it said would be “put to use before long.” The fact that US forces in Korea were hoarding some of the vast sums of money paid by the South Korean government first came to light in 2008, when negotiations were underway for the eighth cost-sharing agreement. At the time, the Ministry of Defense had said that 1.12 trillion won had been accumulated by the US army. Over the past five years, 381.3 billion won of that has disappeared. However, US forces in Korea are not responding to the South Korean government’s request to clarify how this money was used. [Hankyoreh]
You can read more at the link, but the money is likely being stockpiled in anticipation of whenever the big move to Camp Humphreys takes place. Moving all the servicemembers from Yongsan and the 2nd Infantry Division to Camp Humphreys is not going to be cheap. However, if you are a North Korean spy looking to create discord between the Korean public and the US military this is a great issue to demagogue.