The revival of the trade has touched the hearts - and wallets - of millions, particularly in America where schoolchildren have given up their lunch money to redeem Sudanese slaves.
But there is another, hidden, side to this seemingly noble endeavour. According to aid workers, missionaries, and even the rebel group that facilitates it, slave redemption in Sudan is often nothing more than an elaborate scam.
Some genuine slaves have been redeemed - nobody can say how many - but in other cases the process is nothing more than a careful deceit, stage-managed by corrupt officials of the Sudan People’s Liberation Army (SPLA). "The racket is there, right from the top," SPLA official spokesman Samson Kwaje admitted last week. "The money comes from those American kids. But who gets the cheque?"
There is no doubt that the Khartoum government, which has been fighting the southern SPLA rebels since 1983, is a notorious human rights abuser. The rebels have mainly traditional and Christian beliefs while the government is dominated by Muslim extremists.
Neither is there any doubt that it has deliberately rekindled the slave trade by arming a horseback militia known as the Murahaleen, as a counter-insurgency measure. For almost 20 years the Murahaleen have raided front-line villages in rebel areas, murdering, looting and taking thousands of women and children into bondage in the north.
Since 1995 the Swiss-based charity Christian Solidarity International (CSI) has sought to combat slavery by buying the freedom of over 64,000 ‘slaves’.
In theory, CSI arranges for northern Arab traders to buy up the slaves and secretly walk them across the front line to the safety of the rebel-held south. Then CSI flies in, pays the fee per head - between $35 and $50 - and the slaves walk free. Or so it seems.
In reality, many of the ‘slaves’ are fakes, rounded up by SPLA officials to pose for the cameras. The ‘slavers’ are also fake, sometimes a light-skinned rebel soldier that resembles an Arab, other times a passing trader.
Before the CSI plane lands, the children are coached in stories of abduction and abuse to be repeated when a redeemer, or visiting journalist, asks questions. Interpreters may be instructed to twist their answers.
Italian missionary Father Mario Riva witnessed a CSI redemption in the late 1990s, between the towns of Marial Bai and Nyamlell. He was like any other Western observer, but with one difference - he had lived in Sudan for over 40 years and so knew the Dinka people and their language.
John Eibner, a CSI official and the driving force behind slave redemption, was standing under a tree with some slaves. Father Riva recognised them as his own parishioners. "The people told me they had been collected to get money. It was a kind of business," he said.
A rebel official was translating between Eibner and the slaves. According to Riva: "The white man would ask one thing and they would translate something different to the people." Riva did not voice his concerns to CSI, for fear of upsetting rebel soldiers nearby.
But although the slaves are fake, the money is very real. After the CSI plane takes off, the profits - sometimes over $300,000 in one week - are divided up. A small cut goes to the ‘slaves’ and the ‘traders’ but the lion’s share goes to local commanders and SPLA figures. One commander in the front-line Bahr el Ghazal region, Paul Malong, is widely said to have earned enough from slave redemption to buy 40 wives.
John Eibner denies that CSI has been duped. "The money involved is well publicised... But we have our own mechanisms in place to ensure there is no fraud."
Baroness Cox, who split from CSI to form her own redemption organisation, Christian Solidarity Worldwide, in 1997, also denies being cheated. "The slaves’ stories ring true in terms of all the details… It is just horribly true and horribly authentic."
CSW spent more than 100,000 redeeming 2,200 slaves between 1997 and 2001. It has since stopped the practice because, according to a spokeswoman, the organisation has "succeeded in raising the profile of slavery internationally".
But some aid workers in the south see things differently. One nurse with a European agency recalled seeing a slave redemption in late 1999 being carried out by a small American Christian group. "They brought the kids to be redeemed to a clearing under the trees. I knew two of them by name," she said. "They were wearing our [feeding centre] bracelets."
The nurse wanted to alert the Americans that they were being conned but her colleague told her to keep quiet. "He said, ‘there are guys here with guns. Let them give the money if they want’," she said.
Aleu Ayieny Aleu, a retired SPLA commander, alleges that a relative, Captain Akec Tong, has been "forced several times to pretend to be an Arab and simulate the sale of free children". Redemption has become a "racket of mafia dimensions", he said.
CSI estimates that there are 200,000 slaves in captivity in Sudan. Save the Children, which works with enslaved children in the north, puts the figure at 7,000.
The redemption of slaves has become a major US issue. "I thought it was good to give up my lunch money to free slaves," 12-year-old New Jersey schoolgirl Laquisha Gerald told a reporter last year after raising $44 to redeem Sudanese slaves. "We’re doing something good."
But Sudan activists have long feared that a redemption scandal could blunt much-justified criticism of the Khartoum government. Last week a government helicopter fired seven rockets into a village in the south, killing 17 civilians.