Elindholm...or anyone else who knows the answer...
...how are trades involving Diaw (who becomes a BYC player) affected if he is traded to a team under the cap?
I'm about 95% sure this is right, but someone may correct me:
A player's BYC status matters only to the team that is trading him away, not the team that is receiving him. So, for salary-calculation purposes, Diaw's new team would be taking on a $9 million salary. Just as with any other trade, they'd have to either have that much room under the cap or be simultaneously trading out enough salary to get within the 25% fudge factor.
For Phoenix, however, they get to count only half of his salary, so $4.5 million, as "outgoing." That means that, since they are over the cap, they could take back only ($4.5 million) * (125%) + ($100,000), or around $5.7 million.
This is why trades with BYC players are so difficult. If the partner team is also over the cap, they have to send out enough salary to make room for Diaw, which would mean about $7.1 million under the fudge factor rule. But if Phoenix can take only $5.7 million back, there's no way to make the numbers agree.
It's easier if the partner team is under the cap, if more players are involved in the trade (to bring the numbers higher and thus the percentages closer), or (sometimes) if a third team is involved.