Stealing with the thieves

I have agreed with a thief to steal a house, but when I talk to him outside the splurging sturgeon he says that I have to have my party gone. I have tried sending them to the tavern and talking alone to him, but nothing´s changed. What should I do ?

I've taken that quest exactly once, here's why:

*****SPOIL*****

The thief wants you to stand guard while he robs the house with his companions. BUT: if he SEES your companions, he won't react until you return to the thieves guild, where he'll attack you.
If you want to see the quest through, you'll have to enter the map from the west, and let only your thief walk up to him and talk to him. Your thief then has to stand guard in front of the building the rogue takes him/her to. A Flaming Fist soldier will appear shortly afterwards and talk to your thief, even if your thief is invisible or hidden. It's part of the plot. Your thief then has a choice to lie to the Fist and get away (-> you'll be attacked in the guild) or else warn the rogues inside the building and face a hostile Fist. Remember: killing a Fist soldier is really bad for your reputation.