So, what happened here is, I made a blind-offer to someone for a card in their collection. Upon review they agreed and picked one of my cards. (A card I had literally seen 24 hours earlier.) But when I went to look for it, I had mis-placed it, so I let them know immediately. Before I found my card, (24 hours later), I had actually acquired the card I wanted by other means. So I gave the other person the option to complete the trade, or cancel.
In this case, a 1-1 deal will be pretty easy to modify in the DB, but I figured it would be a good learning opporutnity too.
What will the *Problem" option in Transactions do? sounds like it might work here, but I wouldn't want anything to negatively affect either of our ratings etc...