Trade exceptions can't be added to anything. A team can acquire 125% of the salary they trade away (in a simultaneous trade). So if they have a $10 million player, he can be traded for up to $12.5 million in incoming salary. If they also have an outstanding trade exception for $5 million, they can't add it to the $12.5 million in order to acquire a player making $17.5 million. The trade exception can only be used to acquire a player or players making the amount of the exception or less (plus a $100,000 fudge factor).