Hmm, there's been a lot of people complaining that they can't buy a house at Dawnstar house after starting or completing the Dark Brotherhood.
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Sometimes, (there is a glitch) completing "Waking Nightmare" will cause Jarl Skald to lose all dialog options except for the two ones mentioned above.
The only way is to complete the giant killing quest before completing "Waking Nightmare"
This also makes Heljarchen Hall unavailabe (if you have the Hearthfire Addon), as long als Skald is the Jarl of the Pale.
As a "workaround" you can only side wtih the Imperials in the civil war and conquer the Pale. The new Jarl will offer to purchase land.
You wanna read this too:
Spoiler http://www.uesp.net/wiki/Skyrim:Heljarchen_Hall
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There are also, two possibilities - It may be possible to buy the land by just completing the Nightmare Quest or It may be possible to buy the land without doing either of the quests.
It's completely random actually...