In exchange mode, you can only exchange items if they are within the visible scroll area, as it is impossible to view items owned by an ally or items equipped on them. In any other case, you will have to replay the scenario. (Although, it's an interesting tactic: the opponent attacks an ally, and you take revenge later. This is especially true since the AI always runs away, and you don't get anything, although having War Chains would certainly help, if you can find them first.)