Agreed, but there is no requirement or right that the GA engage in the negotiation. If the GA thinks $500 will get the needed seat, he doesn't need to offer the OP anything else. He can open the offer up to the passengers at the gate, and will likely get volunteers. The OPs expectations that DL must negotiate with him and that his seat/flight is worth as much as another flight's seat are unrealistic.

Maybe the GA misspoke about what was happening at other gates. Maybe the GA thought that was the case, but something changed at the other gate. All of that is irrelevant.