I never realized this because it does not have a background and does not look like you can click on it.

I had tried everything, shift-control-clicking on the Transaction Detail slider to try to trade more granular amounts of cargo, using arrow keys...

But you can just click on the number and then type in exactly how much to trade ("9" for example)

This is very nice. Wish I knew it sooner -- maybe the number should get a little bit of a shaded background or border so it is clear that you can click there and manually enter a value.