Step 1: Capture a civilian.
Step 2: Make peace.
Step 3: Move the civilian next to a barbarian.
Step 4: Capture it back and select the option to return to original owner.
This happened to me by accident when I yoinked a settler from an AI and got ambushed by barbarians a few tiles away from my capital. By the time I got it back, I didn't need the worker any more, so I gave it back.
It seems like a highly exploitable technique.
Step 2: Make peace.
Step 3: Move the civilian next to a barbarian.
Step 4: Capture it back and select the option to return to original owner.
This happened to me by accident when I yoinked a settler from an AI and got ambushed by barbarians a few tiles away from my capital. By the time I got it back, I didn't need the worker any more, so I gave it back.
It seems like a highly exploitable technique.