How do I complete the first Thieves Guild quest?

I am currently in the city of Riften in Skyrim. I'm working on the quest A Chance Encounter where I've been asked to steal Madesi's ring out of the locked box in her merchant's stall. This quest initiates contact with the Thieve's Guild so it's important that I finish it.

I'm able to get behind the stall and pick the lock without being detected. When it comes to actually stealing the ring, I'm totally stumped. Even while sneaking, I can't seem to get out of sight when I go in front of the box. There is not looting option when I try and take it from behind. Here's a view of the stall:

I have no idea what I'm supposed to do. Short of chugging an invisibility potion, is there any way to nab this ring without getting caught? I'm sure I'm missing something.

Solution 1:

Just figured this out. You're not supposed to steal from the case on his stand. Go behind it and look down. There is a lock you can pick behind and under the stand and his strongbox is inside this area.

Solution 2:

You can simply steal the ring at night, when no one is there.