What they were taking part in was a corrupted example of the way some Sudanese families are forced to redeem children or other relatives who have been abducted in the course of the inter-tribal raiding and conflict that has spiraled in Sudan because of the continuing civil war. Outside those areas controlled by the Sudanese Government, the old practice of inter-tribal feuding continues. In these raids prisoners are taken, who must then be ransomed. What looks like the purchase of slaves is actually the redemption of prisoners of war. This is a clear factual error.
Christians are a minority even in Southern Sudan.
Christians make up 4 percent of the national population, and some 15 percent of the southern population. Your report showed a slave who had been released two years earlier. This person was quoted as saying the open wounds on his legs were from beatings by his slave masters. In disputed territories this practice was implemented through raids. Soldiers of this same group presumably protected your investigative team.