Sources you requested:
1) The Synod of Melfi under Pope Urban II in 1089
“Wives and concubines were liable to be seized as slaves by the overlord, while the children were relegated to the category of servile rank, debarred from sacred orders, and declared incapable of exercising hereditary rights, because saepe solet similis filius esse patri.”
http://www.fordham.edu/halsall/basis/lateran2.html
Very good, but you forgot to publish the whole canon.
CANON 21
Summary. Sons of priests must be debarred from the ministry of the altar.
Text. We decree that the sons of priests must be debarred from the ministry of the altar, unless they become monks or canons regular.
Comment. To put an end to clerical incontinence various kinds of disabilities were enacted and as far as possible enforced not only against the wives but also against the children of ecclesiastics. Wives and concubines were liable to be seized as slaves by the overlord, while the children were relegated to the category of servile rank, debarred from sacred orders, and declared incapable of exercising hereditary rights, because saepe solet similis filius esse patri. The Synod of Toledo (655) in canon 10 decreed that the sons of clerics in major orders are to be held forever as serfs of the church which their father served .[[30]] In 1031 the Synod of Bourges in canon 8 decreed that the sons of priests, deacons, and subdeacons, born after the reception of these orders, are excluded from the clerical state, because they and all others born of illegitimate unions are stigmatized by the Sacred Scriptures as semen maledictum. They are deprived of all hereditary rights in accordance with the civil law, and their testimony is not to be accepted. Those who already are clerics are to remain in whatever order they are, but are not to be promoted to higher orders. [[31]]. Urban (1088-99) forbade the ordination of the illegitimate sons of clerics, unless they became members of approved religious orders. [[32]]
The present council, following earlier decisions, permits promotion to the ministry of the altar in case such candidates should choose the religious life of approved orders. The irregularity incurred ex defectu natalium is obliterated by religious profession. Moreover, the solitude and enviroment of the religious life, as well as the protection it offers a sufficient guarantee that they will not follow in the sin-stained footsteps of the fathers. From ecclesiastical benefices and from all ecclesiastical dignities they are forever excluded. Religious profession opens the way to sacred orders, but it does not unseal the gateway to dignities or even to regular prelacies.
The purpose was not to have slaves; the purpose was to stop the pilferage of Church property under the pretext of inheritance.
There were a handful of instances of apparent Church sanctioned slavery, but generally relegated to Innocent VIII and Urban VII. You'll find that most popes spoke out against slavery. The Dominicans in the New World vehemently opposed the instances of slavery by the Spanish, earning for them the enmity of the Spanish kings, but leading to the early eradication of Spanish slavery, as opposed to American, English and Dutch slavery, which continued for centuries.
Prior to that time it was English and Spanish slavery.