Stacks of money pass from the Christian foreigner to the Muslim trader, an exchange anxiously watched by a 13-year-old girl with diamonds of sweat on her brow.
The Sudanese trader, his lap buried by currency worth $13,200, waves carelessly to free his merchandise - 132 slaves.
Akuac Malong, the young Dinka girl, is among them. She has spent more than half her life enslaved in northern Sudan. Her brilliant smile belies the beatings, near-starvation, mutilation, and attempted brainwashing she endured. "I thought it would be better to die than to remain a slave," Akuac says.
Trafficking in humans has resurged with civil war in Africa's largest and poorest country, says John Eibner of Christian Solidarity International, a humanitarian group that bought Akuac's freedom.
For all but a decade since Sudan's independence in 1956, southern rebels, mainly black Christians and followers of tribal religions, have fought for autonomy from the Khartoum government, which is dominated by northern Arabs. The southerners believe the north is trying to impose Islam and the Arabic language and to monopolize Sudan's wealth.
Since the rebellion resumed 14 years ago, fighting, famine, and disease have killed an estimated 1.5 million Sudanese. More than 3 million people have fled or been forced from their homes.
Much of the fighting on the government side is done by local militias. Unpaid, their bounty is as old as war itself: slaves. Sudan's Islamic leaders encourage soldiers to take slaves as compensation, according to UN investigators and the US State Department.
"According to the Khartoum regime's ideology of [holy war], members of this resistant black African community - be they men, women, or children - are infidels, and may be arbitrarily killed, enslaved, looted, or otherwise abused," Mr. Eibner says.
The government denies condoning slavery, insisting that the practice persists because holding prisoners for ransom is a tradition rooted in tribal disputes.
No side has a claim on morality in this war. The rebel Sudan People's Liberation Army has been accused of forcibly inducting teenage boys into its ragtag army. But southern blacks do not enslave Arab prisoners.
Christian Solidarity International estimates that tens of thousands of black slaves are owned by Arabs in northern Sudan. The Swiss-based charity has made more than a dozen risky, clandestine flights to southern Sudan to redeem 800 slaves since 1995, most recently in Madhol, 720 miles southwest of Khartoum.
Alex de Waal of the London-based group African Rights says that by paying large sums to free slaves, the Swiss charity undercuts Dinkas living in the north who do the same secretive work for a fraction of the cost.
Eibner counters: "There is no evidence to suggest that our work has undermined efforts to redeem abducted women and children. In fact, Dinka elders encourage us to press ahead with our activities."
The centuries-old tensions between Arabs and blacks are linked to slaving expeditions by Arabs to the upper Nile, a trade that 19th-century explorer David Livingstone called "an open sore on the world."
Akuac's mother, Abuong Malong, sobs when she sees her daughter for the first time in seven years. "It's like she's been born again," she says. She recognizes her only from her straight, square teeth. "She was very small when she was taken, her features have changed, but she came back with the same spirit."