A victorious invasion probably draws an unconditional surrender, where the victim just has to take whatever the victor dishes out. i.e. he succumbs to the invasion.
--but what about a conditional surrender. It would be pretty awesome to be able to call for a parley in the middle of a siege, and have you leader or a skilled negotiator meet with the goblin warlord.
It could be like trading with a caravan , except instead of buying goods, you're paying them not to kill all your people.
You might not be able to do that to the less formal raids that happen early on, but a full fledged siege might be susceptible to such tactics.
This saves the goblins the long wait and the battle with your defenders, and it allows you to lose just your hoard, instead of your hoard, your dwarves, and your fortress. They could even demand the tribute delivered to your trade depot, in order to recycle some code.
On second thought, they should provide a depot like place to deliver the goods. Knowing the kind of death trap some of the trade depots can be, that would make it too easy to kill the team sent to collect the tribute.
BTW: Nice derail Backpak! That's some nice trolling, what with the framing of somebody else, built right in. /sarcasm