But for Rim Il, the deal soured from the start: Under a moonlit night, the bus carrying him and a score of other fresh arrivals pulled into a desert camp cordoned off with barbed wire fences.

There, 1,800 contract workers, sent by North Korea to earn badly needed hard currency, were living together under the watchful eyes of government supervisors, Mr. Rim said. They worked from 7 a.m. to 7 p.m., seven days a week, doing menial jobs at construction sites.

“We only took a Friday afternoon off twice a month but had to spend the time studying books or watching videos about the greatness of our leader back home,” Mr. Rim said at a recent news conference in Seoul. “We were never paid our wages, and when we asked our superiors about them, they said we should think of starving people back home and thank the leader for giving us this opportunity of eating three meals a day.”