I agree. I rewatched it in slow motion and freezing some of the frames. It's definitely Willie's shoe, and the thug still has both of his. Would be too much of a silly coincidence for two people to lose a shoe. And I think your explanation that he didn't want to leave any evidence is a good one. That a small thing such as this may have major consequences fits in very nicely with the existentialist spirit of the film: losing a shoe leads to a witness being left alive (the thug who turns to get it is later questioned by the police etc).

It was Willies shoe; check out the scene when they are in the apartment with the Australian guy (at 29 mins). Willie only has one shoe on. They also buy him a pair of trainers later in the film.
The guy who goes back to pick up the shoe is wearing blue jeans, whereas the close-up shows someone who is wearing white trousers lose their shoe. Still doesn't explain why he went back to retrieve the shoe, unless he didn't want to leave any evidence behind.

It
wasn't
Willie's shoe. A close-up shot clearly shows the shoe coming off the thug's foot. (In any case, why would he go back for Willie's shoe after Willie has already been driven offrather than grab his share of the loot like the rest?)

No doubt, it was Willie's shoe.

No, it was Willie's shoe.

i just watched this film. the thugs take the briefcase, get in the car. one jumps out to retrieve HIS shoe(not willie's)as the others are saying 'we're going to be rich' and the car blows up.
Ad hoc, Ad loc, Quid pro queeee,
So little time and so much to see

yeah, he did. you were right.

Doesn't he turn back for the shoe that Willie dropped?