This page of the guide The Elder Scrolls V: Skyrim features the beginning of the walkthrough for the quest A Chance Arrangement. This is a brief introduction to the main part of the quest. The only required step is to talk to Brynjolf, although it is necessary to do it during the daytime.
New main objective: Meet Brynjolf during daytime
Note! This is a side quest which you can complete during A Cornered Rat. If you managed to use Speech and persuade Brynjolf to tell you the whereabouts of Esbern's hideout or if you found the old man yourself, you won't need to complete it.
You can meet with Brynjolf at the main square of Riften and in accordance with the information he gave you it can take place only during daytime, i.e. between 8 a.m. and 8 p.m. If the hour isn't right, you can use any bed or just wait until morning comes. In the latter case, press T and choose how many time should pass. Find Brynjolf (screen above) and initiate a conversation to begin the whole action. He should call the other traders to him, thanks to which you will be able to begin.