Gniezno Cathedral Door - detail. Saint Adalbert of Prague pleads with Boleslaus II, Duke of Bohemia, for the release of Christians slaves by their masters, Jewish merchants.

Diversity Macht Frei, “Christian slaves, Muslim masters, Jewish slave-runners: ‘The slave-dealers in Europe were almost all Jews”, 23 Feb 2018:

In Islamic slave markets, all races were not considered as equal. Negresses were held in particularly low regard.

“At the markets negresses were much in evidence; the darker the uglier and the more pointed their teeth,. They are not up to much. They are fickle and careless. Dancing and beating time are engrained in their nature. They say: were the negro to fall from heaven to the earth lie would beat time in falling. They have the whitest teeth and this because they have much saliva. Un-pleasant is the smell emitted from their armpits and coarse is their skin.”

White slaves were the most highly prized of all.

As regards prices, the white slaves—aristocracy of the slaves—stood on quite a different footing. A good-looking, but untrained, white slave-girl fetched 1,000 dinars or more.

Jews were principally responsible for trafficking white Christian slaves to Muslims.

In the rest of the empire the supply of white slaves was confined to Turks and members of that inexhaustible race which has given the caste its European name, “the slav(e)s.” The latter was rated higher than the former as merchandise. Says Khwarezmi: We take to Turks when no other slaves are available. The chief article of export from Bulgar—capital of the Volga Bulgarians—was slaves who were thence taken to the Oxus’. Samarkand was the greatest slave-market noted for the supply of the best white slaves, and depending like Geneva or Lausanne of our time on its educational industry. The second channel of import for slaves of Slavic nationalities lay through Germany to Spain and the Mediterranean harbours of Provence and Italy’. The slave-dealers in Europe were almost all Jews. The slaves came almost exclusively from Eastern Europe as is the case today with the “white slave traffic.”

…Finally the third route for the slave trade likewise led from the western slave countries, then at war with Germany and consequently productive of human merchandise, to the East over Prague, Poland and Russia—a route followed by Rabbi Petachja in the 6th/12th century. In the 4th/14th century Prague was the starting-point, being a centre of the then slave-trade. Saint Adalbert gave up his bishopric in 989 A.D. because he could not redeem all the Christians whom a Jewish dealer had purchased.

Source: The Renaissance of Islam by Adam Mez (link)

The role of Jews in the traffic of Christian slaves to Muslim lands is confirmed by a letter Bishop Agobard of Lyon wrote to his king, Louis the Pious, in 826. Titled (“On the Insolence of the Jews“), it was a complaint to the king about Jewish slave-running and general arrogance.

This was during the period of Muslim domination of large parts of Spain. Agobard himself had been born in Spain. He notes that Christian slaves were being sold to the Muslims there by Jews. Although these slaves were probably trafficked from elsewhere, Agobard also records example of local Christian children being kidnapped by Jews and sold to the Muslims.

The Jews began to rage with a certain odious insolence, threatening that we would be afflicted with every sort of injury by the agents whom they had obtained to take vengeance upon Christians. After them, Evrard arrived and repeated the same thing and said that your majesty was truly angry with me because of the Jews.

…We suffered these things from the Jews’ supporters and for no other reason but that we preached to Christians that they should not sell Christian slaves to them; that they should not allow these Jews to sell Christians to Spain nor to possess them as paid domestics lest Christian women celebrate the Sabbath with them, work on Sundays, eat with them during Lent, and their paid servants eat meat on these days; and that no Christian should buy meats sacrificed and butchered by Jews and sell them to other Christians; and that they should not drink their wine or other things like this.

[…]

After the preceding note had been dictated, a certain man from Cordoba arrived, fleeing from Spain. He said that he had been stolen as a little boy by a certain Jew of Lyon 24 years before and sold, and that he had fled this year with another boy from Arles who had been likewise stolen by a Jew six years earlier. When we sought out those known to the man who was from Lyon and found them, some said that others had been stolen by this same Jew, others bought and sold, and that this year another boy was stolen and sold by a Jew. At that moment it was discovered that many Christians are sold by Christians and bought by Jews and that many unspeakable things are perpetrated by them too foul to write.

Source: On the Insolence of the Jews by Agobard of Lyon (link)