You don't sell someone something they already own. *G* The Black Lance built the Dragons with Confed funds, most likely diverted from the black ops budget or some other type of untracable slush fund. While I think it's a bit much to say that the Border Worlders "stole" them (any more than they "murdered" the people they shot down, as the normal rules tend to go out of the window in a war, declard or otherwise), the fact remains that they were Confed property to begin with. Demanding a ransom for their return wouldn't have done much for Confed/BW relations, which weren't all that good to begin with.
Having said, I never said that the BW would *just* hand them over. Like you say, it seems normal practice to have a good look at high tech before you hand it over. Not only that, international negotiations usually work on a quid pro quo basis. If the Border Worlders played nice and let Confed have its Dragons back without making a fuss, then Confed would most likely reciprocate in by giving way on something that the Border Worlders wanted. Both nations get what they want, the wounds of the recent Confed/BW conflict don't get re-opened, and everyone's happy.
PS: Seeing as this *is* the fanfic board, I might as well tie this into HTL.
The Border Worlders' Retaliator fighters, which you can see in action in the current chapter "On Deadly Ground" are very heavily based on the Dragons.
Best, Raptor