I'm going to share some quotes from an article titled Roman Labor in Transition: Slaves, Coloni, and Other Workers form the Saylor Foundation.

In the late Roman period, slaves became rarer and more expensive... with the end of Rome’s expansion and conquest of new lands, the empire was no longer taking in large numbers of enslaved captives. When slaves had been very cheap, they were used extensively in agricultural work, making up the workforce of the great latinfundiae, or plantations, of the late republic and early empire. But as slaves became rarer and more expensive, they tended to be reserved for work that would bestow more prestige on their owners, such as working within the household. Some great landowners continued to use slaves to work their land, but most preferred to lease their lands to poor farmers and collect rent on the produce.

As slaves became more valuable, they enjoyed a modest increase in their rights and status. Slaves were assigned plots of land and would live off of the produce while paying their master a certain percentage of that produce. The Roman government passed a law in the middle of the fourth century that prohibited selling an agricultural slave apart from his land. Thus, the slaves were tied to their land the same way a tenant farmer might be. Slaves were allowed to marry free tenants, to acquire property, and to pass their property on to their children.

Still, this does not mean that all slaves enjoyed increased benefits or freedoms. Household slaves and domestic servants saw little change in status, nor did the slaves who labored in difficult jobs in workshops and factories. And while slaves may have been rarer than in early imperial times, they were still quite common. Many people were enslaved from barbarian peoples on Rome’s borders, and on the frontiers slaves seem to have continued to be cheap. Even humble Roman households would usually have one or two slaves. All officers in the army had personal slaves, and the wealthy aristocrats of the empire had large teams of slaves. Even with the fall of the Western Roman Empire, slavery continued to be an institution in early medieval Europe. Slavery was not replaced by serfdom, but rather, the two institutions existed side-by-side.

The article also describes how coloni, free tenant farmers, transformed into a slave-like condition similar to the agricultural slaves above described. The agricultural slaves and coloni, to succinctly generalize, transformed into serfs, while the household slaves remained slaves as in Roman times.