Ok.. I reckon i've done this quest about 5 times with different character but i dont remember seeing that..
Is there a way you can go back and ask to buy it after you've completed the quest?
Talk to the Jarl's Steward after completing the quest for the Jarl.
The other gotcha is that the Jarl's Steward must be standing near the throne and the Jarl for him to have the option available.
I've done this a few times, he keeps some odd hours, I have the most luck around 8-10PM.
Dan O.