U.S. citizens and residents must pay tax on their worldwide income. There is a limited exclusion for earned income (up to $103,900 in 2018), but there is no exclusion for gain on the sale of property abroad.
Debt on which a person is not personally liable. In case of nonpayment, the creditor must foreclose on property securing the debt. At-risk rules generally bar losses where there is nonrecourse financing, but an exception applies to certain nonrecourse financing for real estate.