Remove items from trading window?

I've seen others do it - I just can't figure out how

I show someone two items - they want one, all I want to do is remove one without having to cancel trade and re-enter. I could have sworn someone has done this with me before. I've dragged, hit delete - nothing.

click "modify" and then double click on the item you want to remove from the trade window.

or drag it up into the black area above ur trade section

Thank you both. I was trying to drag back to my bag, double clikcing just didn't occur to me!