Manumission of slaves was quite common in Rome
Manumission or freeing a slave had nothing to do with moral considerations. A slave was considered an instrument with a voice. Normally a slave would be as lazy as possible, but if there was a prospect of manumission for talented slaves, they were more willing to give their best.
There was no moral consideration, but it was seen as a reward for services rendered or the owner felt the slave to be too talented to keep him in slavery.
After the disaster at Cannae was a great shortage of men in Rome. In order to rebuild an employable army, usable slaves were bought and promised freedom after a period of three years good service in the army.
Another reason for manumission could be the prevention of a slave being questioned in a trial against the owner. When a slave was questioned, that slave was to be tortured according to the law. A free man could not be tortured. This was of course also a form of self defense of the owner, who feared that his slave could incriminate him, when he couldn't’t endure the In 337 bc the Pontifex maximus prevented an accused vestal to manumit her slave. Later a manumission like this was formally forbidden.
Since many slaves were allowed to have property (Though formally that was still property of his owner) and income, it was also possible that a slave bought his own freedom, if his master would allow it.
Very generously a freed slave was immediately given Roman citizenship.
T. Livius (2.5)
(After the slave has disclosed a treason to bring Tarquinius Superbus back to power)
...the informer was assigned a sum of money from the treasury and he was given his liberty and the rights of citizenship. He is said to have been the first to be made free by the "vindicta." Some suppose this designation to have been derived from him, his name being Vindicius. After him it was the rule that those who were made free in this way were considered to be admitted to the citizenship.
Citizenship yes, but a freedman could not opt for a public office. But
his freeborn children held the full citizenship.
This was a typical Roman habit the Greek Dionysius of Halicarnassus(4.24) considered the habit (like most Greek did) as evil. Interesting is his remark:
The Romans acquired their slaves by the most just means;
Which emphasizes again the fact that slavery was accepted as normal.
Once freed the former slaver became automatically a client of his
former owner and assumed his praenomen and nomen and his own
So we find on a Roman headstone:
Here lie the bones of
Quintus Tiburtius Menolavus,
freedman of Quintus,
who slaughtered animals for sacrifice.
What was an obligation for slave women, was expected of freedwomen and absolutely forbidden for free Roman women. Having sex with a slave woman was not seen as adultery.
When a son, sold by his father, was freed he would return in his father’s potestas. Then his father could sell him again. When this would happen three times, the son would become independent from his father.
Later this habit of manumission would bring social problems to Rome as there were too many freedman. The granting of full citizenship would be limited. See Gaius institutes 1 par 14-46 Second and even third class were introduced: Latins and dediticii. The latter being captured enemy soldiers, who would get very few rights when freed. Manumitting a slave who was under thirty was in principle not allowed. Freeing slaves by will became restricted, only a certain part of the slaves could be manumitted.