At the end of Following the police says that they found one of the earrings in Bill's shoebox. But he never put it there. If Cobb put it there, why didn't he simply give it to him to take it home in the first place?
For your original question:
- Of course Bill did not take it because he did not find it at 00:27:52 or later.
- The earring was put there to pin the killing of Blonde on Bill. So it couldn't have been blonde who hid it either.
- So Cobb must have done it, but we see at 00:11:23 that Cobb fails to pick the lock and also later at 00:29:48 we see that he breaks the door, so he definately can't pick a lock. Therefore, if he wanted to break in Bill's house he should have broken the door and so Bill would have noticed. So it is only possible that he did it at the end of the story, supposedly when he takes the money from his house, but then the police would suspect someone else must have been involved who broke his door and took the money. But they could also think that he hid the money somewhere else and broke his own door to backup his story.
As for your second question:
Because if he did that, Bill might have given it back to the Blonde, but Cobb needed it to be found in Bill's house. We know that he suspects Bill would do that because
You may even get most of it back, if you're lucky.
So does that mean you'll tell me where you hid my earring?
- No. And I wouldn't wait for your panties either. He's too embarrassed to give those back to you.