I recently finished reading Snyder's *The Britons* and have a question regarding slavey. On page 149 the book says:
"Sidonius [a "Gallish aristocrat"] ... complains [in a letter to a man named Riothamus] that certain *Britanni* ... were enticing slaves away from the estate of one of his friends...."
The letter in question was written "around 470" which made me wonder about the practice of enticing slaves away like this. It could be that I misunderstand the degree of power slave owners had over their captives, but the ability to leave one estate for another sounds like much more freedom than slaves had in my conception of the period. My immediate read of it was that it might be a sign of change towards the sort of system in the area in the middle ages, but I'm not familiar enough with Roman history to know if this was common earlier.
The source mentioned is Sidonius Apollinaris Letter 3.9. The text basically says what the summary quotation says: "his slaves are being taken away by Britons inciting them in secret" (mancipia sua Britannis clam sollicitantibus abducta). Since both abducta and sollicitantibus can have violent and persuasive connotations, I've tried to keep the translation language neutral between those options. I don't think this is referring to bandits capturing local enslaved people, but it is possible. There is no mention of other estates, so I don't think the idea of moving to other estates is what Snyder or Sidonius mean; I think they are referring to runaways and potential related violence apparently incited by these Gallic Britons.
Later in the letter, Sidonius mentions the complainant is a common farmer, whereas this is a large, armed group, so he needs a fair hearing to show the truth of the matter (Sidonius admits he doesn't' know if the claims are true). Basically, Sidonius is acting on an enslaver's behalf to get a hearing whether a group has been urging enslaved people to run away (or capturing them).
The enslaved people are not leaving the estate for another, but running away entirely. There was nothing on rural estates that would stop enslaved people from being able to leave, other than the difficulty of escaping a rural area. If this local group were trying to recruit through enslaved people, that might offer protection and enough incentive to go for it. The invasions and loss of formal Roman presence may have also proved an incentive for people to try larger scale uprisings or running away as well. I am not aware of other sources mentioning such a mass movement, so this might have just been a local issue limited to estates with less resources. It was very common for enslaved people to run away in earlier periods, so this sounds like a continuation of that practice with the addition of less regulation of the later period.
Though the market of enslaved people may have been in decline in this period, it still existed. There was also the system of coloni, people not-quite-enslaved who were tied to specific land to farm, that continued and grew in this period. Basically, landowners still had options for their workforce and didn't need to take people from local estates, which would have been illegal, unpopular, and easily traceable.