The Manhattan trade was unethical and the Dutch knew it[1]. The comparison is not apt, to update it:
Aliens land and offer exchange of a novel metal alloy for your house and its land. They demonstrate the metal alloy has incredible sci-fi properties and its value (monetarily for you and the discovery for all human kind) far outweighs the value of your house. You accept the trade.
What you didn't realize is that the production of the metal alloy is impossible by Earth civilization standards, its production requires the energy of several sun-like stars (this is a trivial amount of energy for the aliens). Also, there was a misunderstanding in the trade, the metal alloy wasn't for just your house and its land, it was for the entire Northern hemisphere of Earth! The aliens colonize the land backed up with the signed and legal trade deal. Its legality is of course disputed by Earth courts but the alien colonists carry novel viruses which eradicate 80-90% of the existing Earth population, governments and militaries crumble.
Closer: A few aliens arrive, look around, decide it is too hard to live here, then go home. But, they leave behind viruses that reduce the population by 90%. 100 years later, aliens come back in much larger numbers, this time to stay. At that point they offer the alloy.
> Its legality is of course disputed by Earth courts but the alien colonists carry novel viruses which eradicate 80-90% of the existing Earth population, governments and militaries crumble.
This indeed happened in America, but as the analogous example here tends to show, it kind of makes the contract issue moot.
Aliens land and offer exchange of a novel metal alloy for your house and its land. They demonstrate the metal alloy has incredible sci-fi properties and its value (monetarily for you and the discovery for all human kind) far outweighs the value of your house. You accept the trade.
What you didn't realize is that the production of the metal alloy is impossible by Earth civilization standards, its production requires the energy of several sun-like stars (this is a trivial amount of energy for the aliens). Also, there was a misunderstanding in the trade, the metal alloy wasn't for just your house and its land, it was for the entire Northern hemisphere of Earth! The aliens colonize the land backed up with the signed and legal trade deal. Its legality is of course disputed by Earth courts but the alien colonists carry novel viruses which eradicate 80-90% of the existing Earth population, governments and militaries crumble.
[1] 5th to last paragraph but all of it is informative http://newamsterdamhistorycenter.org/education/schaghen.html