"Amount of denied boarding compensation for passengers denied boarding involuntarily.
You see the word "involuntarily", right? That's a rule which applies to "involuntary" denial of boarding. It in no way limits the right of a commercial company to stand at the checkin, or the door, or indeed in the aisle, waving a pile of dollar bills under people's noses asking for volunteers. That regulation, as in the EU, limits the passengers' rights to sue for consequential and punitive damages in the event of refusal of boarding: they just get the defined amount (and no less than the defined amount). It's nothing to do with any deals the airline might care to strike with passengers who volunteer.
An airline can say "we're overbooked, anyone want to get off for a million dollars cash?" without any legal problems. You're trying to claim they are limited in this. You are wrong about this. A passenger who has been _involuntarily_ offloaded cannot sue for a million dollars. You are right about this.