This post deals with the events in last night's episode of A Game of Thrones, and A Storm of Swords.
In the third book of the A Song of Ice and Fire series, Danyrs (Dany) is in search of an army she can use to reclaim her crown. She comes across a slaver with 8000 slave soldier for sale. The Unsullied are noted most of all for their intense discipline, to the point where they will stand night and day without food or water, or fall on their own swords if so ordered. Lacking money, Dany agrees to trade one of her three dragons for all of the slaver's Unsullied. This is of course a boneheaded move for the slaver, because duh, Dany can just kill the slaver and steal the dragon back by using her big shiny new army.
That's not quite how it goes down though. Instead she has her dragon kill the slaver. Here's the relevant text:
Dany handed the slaver the end of Drogon's chain. In return he presented her with the whip. The handle was black dragonbone, elaborates carved and inlaid with gold. Nine long thin leather lashes trailed from it, each one tipped by a gilded claw. [...]
Dany turned the whipe in her hand. [...] "Is it done then? Do they belong to me?"
"It is done," he agreed, giving the chain a sharp pull to bring Drogon down from the litter."
[...] Though the Astapori yanked and tugged, Drogon would not budge off the litter. Smoke rose grey from his open jaws, and his long neck curled and straightened as he snapped at the slaver's face.
[...] "He will not come," Kraznys said.
"There is a reason. A dragon is no slave." And Dany swept the lash down as hard as she could across the slaver's face. Kraznys screamed and staggered back, the blood running red down his cheeks into his perfused beard. The harpy's fingers had torn his features half to pieces with one slash, but she did not pause to contemplate the ruin. "Drogon," she sang out loudly, sweetly, all her fear forgotton, "Dracarys."
The black dragon spread his wings and roared.
A lance of swirling dark flame took Kraznys full in the face. His eyes melted and ran down his cheeks, and the oil in his hair and beard burst so fiercely into fire that for an instant the slaver wore a burning crown twice as tall as his head. The sudden stench of charred meat overwhelmed even his perfume, and his wail seemed to drown all other sound.
Had Dany simply used the Unsullied to kill Kraznys, and then took her dragon back, we'd say fine. Kraznys is an idiot, and Dany is a cutthroat bitch, but all's fair in love and stupidly selling your entire army.
Instead, Dany has Drogon kill Kraznys, giving rise to the arguments that either no contract was formed, or that Dany breached.
The argument that there was no contract rests on the idea that a dragon is not an alienable chattel. Were Drogon a mere pet, he could be handed over, and tough shit for the new owner if he doesn't obey. But, Drogon is a very special type of creature. He follows Dany, even obeys his commands, but it's not clear that she necessarily owns him. He may be more Jorah Mormont. He follows her and obeys her commands, but as a free man, she cannot trade him (without first enslaving him).
So, if Drogon is more like a free person than a pet, no contract was formed due to mistake or lack of consideration.
In the alternative, if Drogon did legally pass to Kraznys, Dany can be argued to have breached by interfering with his taking possession of the dragon. Dany is under no obligation to make Drogon behave, just as someone selling a dog doesn't have to follow along with the new owner and give it commands. But, if a dog seller takes the cash, hands over the leash, and then immediately orders the dog to come, and has it run away from its master and back to the seller, that's a breach. Having the dog breath fire and melt the buyer's eyeballs is a bigger breach.
The Unsullied unanimously accept Dany's rule, with none of them questioning their legal status, and maybe Astapor law doesn't care about these objections, but rest assured that in Winds of Winter there will be Westerosi maesters sitting high in their ivory towers doing the important work of debating the legality of this transaction, and deriding the work of those members of the order who actually work as physicians and medical advisers.