Lo-Fi Games

When I sell a slave to the slavers, his guards attack me, accusing me of kidnapping the carried slave.
This has only just started happening, and like my other cannibal bug report I have disabled all mods and started a new game and it still happens.

I've seen in the 0.98.39 patch notes it says...

"-Slavers notice if you try stealing slaves by carrying them away"

they seem to be noticing you are carrying their slave as soon as you sell them, before the boss has chance to take them.

Was the person you were selling a slaver?
Cause they don't like it when you sell them their own men.
I made that mistake once. ONCE!

Nope, was a rebel farmer, tried it again with a starving bandit. They were fine with me carrying them around until I clicked "Deal"

I assume it is related to a faction or squad switch when you sell.

I do run a lot of mods, so I exited the game, deselected all mods, started a freedom seekers start and immediately picked up and sold one of my own men while carrying him, again the slavers turned hostile when I clicked to sell.

Slaver dialogue in the dialogue tab said "you're not taking my friend away" or something similar, so it is triggering the dialogue as if you'd kidnapped a slaver.

I guess you've coined it, carried person's faction is changing before slaver can pick them up and they get angry. What you can try as a workaround is putting them into a cage manually, i think this way you should still get your money, and this should keep you save from any hassle. At least this was working with bounties at some point.