I would return the extra bread.
Why wouldn't I keep it? Maybe because I think about far too many possibilities. What if the bread seller has it just as worse as I do? What if he's only one day from losing his business?
Maybe that won't happen, but it's a possibility.
And I'd rather have the trust of the bread seller than to have just extra bread for the night. Think about it, if I were to give back the bread, he would then realize that I am trustworthy enough to make up for his mistake. Really, a lot of people would just take advantage of the opportunity, but as a result they would have lost a possible connection with the bread seller.
Maybe in the future he could be a benefit to you just because of that action, sometimes even smallest actions amount in the largest reward. Oh and things do become habit, so if you take the bread, think about that coming a habit in the future, vice-versa.