The Drummond-McCollum rumor makes no sense, or implies that the Pistons don't have anyone dedicated to understanding the cap.
McCollum signed a $106 million extension. If he's traded, he counts only his current year salary for the Blazers in calculating how much they're allowed to take back -- so 50% more than his $3.4 million salary, or about $5 million. Drummond makes over $22 million. For the Pistons, McCollum counts as the average of this year plus all the years of the extension, or just under $22 million. So one of two things is true with this rumor:
1) The Pistons don't know this salary cap rule.
2) They never asked for McCollum.