I went into trade mode with a trader, however my in-focus character was a companion. It went into the trade dialog, but when I left I was looking at the companion with no option to leave. The buttons at the bottom left allowed me to go into trade again or change characters, but there is no leave. I tried changing characters to no avail. It may be that switching characters inside trading caused the problem. If I switched to the player-character, I could walk away, but the companion who was trading was still stuck in trade mode.