I can confirm that you need to do favors for the Jarl in Falkreath to purchase the land, and when I would try to talk to the Jarl, I wouldn't get any options to do quests for him. I did have the Mammoth Tusk quest for Ysolda, so I finished that and went back to the Jarl and his quests appeared. I finished the two quests and was able to buy the land.
Hope that helps.
To be even more clear, his first quest is retrieving one bottle of Black Briar mead from Riften, and the second involved killing Bandits at Bilegulch Mine.