When you drag the salt over the coal, both items should disappear, with a new item (the mixture) going into the hero's inventory. If you are mixing them in the DK's inventory, try moving them to your hero (mixing should work with the DK, aside from the result being given to the hero).
Also, make sure you are combining these items in the main game, and not in the BF.
Not that there shouldn't be a confirmation when hitting Enter to accept a transaction, but AFAIR you are the first to discover/mentioned this.
You can safely cancel a trade by hitting Esc, right clicking the window or clicking the cancel (X) button.
