You will first have to determine if that country has a double tax avoidance treaty with US. It is possible that based on that treaty gains from sale of such property will be taxed only by the country where the property is located. Hence you may not owe any tax on the gain from sale of such property in US. And, the child can bring the money is US.
If it is subject to capital gain tax than your original cost basis in the property plus any gift tax paid by the donor is considered as the cost basis for the recipient of the gift.
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Please note: This advice is provided with the understanding that all the relevant facts have been provided by you. Any change in facts might affect the advice given and hence may not be relied on in such cases.
US-Greece do have a tax treaty (see link below) however, it does not cover capital gains.
Hence, gains will be subject to tax in US. This will be long term capital gains in US taxed upto a maximum of 15%. You will also get a credit for taxes paid in Greece on this income in US and hence your US tax liability on this gain will be lowered to that extent.
Original cost basis is calculated based on what the parents paid for when they bought the property. Parents do not pay gift tax now as the gift took place long back and as it is gift upto $ 1 mn. are not subject to gift tax at they can be excluded(life time exclusion). But they were required to file a gift tax return when they gave the gift if they were US citixen of resident at the time gift was given.