At first, when Charlie goes to buy candy with his found money, he just wants the biggest bar he can get.
The candy store owner gives him a random Wonka bar.
Part two of the theory argues that the candy store owner where Charlie buys the chocolate bar is actually an undercover employee of Willy Wonka, similar to the guy who claimed to be Slugworth.
After the fake ticket story goes out, he’s instructed to give the bar with the ticket to the right kid, probably somebody poor, but who has a good heart.