It's exactly as TomD8 said as far as I can tell.
Basically, the best option is to take the tax amount paid in South Korea (the 30%) and apply a foreign tax credit for the IRS federal tax. This credit will be weighted by the portion of the foreign income / total income for this year, and only that portion can be used to offset the capital gain taxes in the US.
Since the rate was 30% in your case for South Korea and 25% in the US, let's take a simple example.
You sold your property for a profit of 100, and paid 30% tax on it, or 30.
Meanwhile, in the US, your revenue was 200, so your total revenue for this year was 300 and the ratio of your foreign income is 100 / 300 = 33%, so only 33% of the 30 paid as taxes in South Korea can be used as a foreign tax credit for the IRS. The remaining 20 can be used as credit one year back or 10 years forward apparently, but again, only for the purpose of foreign tax credit.
With a capital gain rate of 25%, your tax amount would be 25, and you can apply 10 from the South Korean tax credit, and you pay an additional 15.
And then, there is the question of the state taxes. Unless you live in a state where there is no state tax, they will request a tax revenue on any amount your earn worldwide.
