What I did
- The skooma questline.
- 5 Misc quest from riften citizens.
- Collected the 8000 Gold.
After that, the Jarl told me that I can buy a house from the steward.
If I talk to the steward I have the option to buy the house but if I click it, the only option I have is saying "I'd rather wait some time and think about this decision again". I already tried waiting a long time. But that didn't help.
I'm level 13, if that matters.