How do I sell my prisoner into slavery?

I have a prisoner that I don't want to care for, and a slave trader in comms range. How do I sell the prisoner to the slave trader? I can see the prisoner show up in the trade screen, but I cant figure out how to actually sell him.

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Solution 1:

Drag the quantity in the "Drag to trade" column towards the slave trader, the same as you sell any other goods to traders.