The Norse had a specific conception of slaves, different from most, if not all, other societies with slavery. These slaves were known as thralls.
Now, as I explain thralls, don't forget that we're talking about slaves; humans bought, owned, and sold as property, belonging to the lowest class in society, with little-to-no rights to speak of.
Thralls generally lived a better life than other contemporary slaves in Europe, not that that's a high bar. While they didn't possess any legal rights themselves, masters would usually maintain some sense of rights they would possess, and in turn, these could be legally recognized as being rules regarding said master's property.
It was a fairly common practice, but not the norm, that thralls could either earn a “wage" of sorts through their labor through which they could purchase themselves eventually, or for masters to grant thralls the ownership of themselves in their wills.
If a thrall gained ownership of themselves they wouldn't however become a freeman — they'd become a freed thrall, a semi-class between thralls and freemen. Their children, if they were to have any, would, in turn, be the children of a freed thrall, another semi-class between thralls and freemen, and then their children, the grandchildren of the original thrall, would be freemen. So the process of becoming an actual person took two generations.
There are plenty of legends that supposedly depict how well-treated thralls were, being allowed to voice complaints and being allowed breaks and such, however, I do not know the legitimacy of these legends.
One example of how the Scandinavian church recognized thralls as having some rights exists though and is believed to be true. A son of a noble, named Hallvard, was murdered while protecting a thrall woman who was being accused of stealing without any evidence. Because of this, he was canonized as Saint Hallvard, patron saint of Oslo, by the church.