As one of the loadscreen hints says, returning (giving as gift) prisoners to their faction will be highly appreciated.
So I organized a caravan to the prisoner's faction settlement, and given that prisoner as gift. Got a nice boost to relations, which is good. Also my colonists got "prisoner sold" (prisoner sold into slavery... as tooltip says) mood debuff, but I haven't sold him, I returned him to his faction, so that debuff should only occur when prisoner is really sold, not set free.
Just a question, do you remember how much standing did you got when you return the prisoner at this way ?
More then you would get when you simple release them ?
But i agree, the mood debuff is wrong it should be free prisoner instead.
+12 for releasing healthy guy, something like +34 for delivering him to faction settlement.
Social debuff (-15, people dislike guy, who returned prisoner to his faction, think he rather sold prisoner into slavery) is wrongly given in this case too.
It's a known issue, thanks for reporting.