When Ben, Riley, and the Decleration lady(I forget her name) are in the store, Ben sells his Watch for $100. After he looks at the $100 bill and walks out you can see him putting his watch back onto his wrist!!
The watch was given to the cashier as collateral while they were looking at the $100 bill. When they were done with the bill, it was given back to the cashier and the watch was returned. No mistake or slip-up here.
I don't believe he SOLD his watch; rather, he used the watch as collateral with the sales clerk in order to to inspect the $100 bill he had just used to pay for his clothing. After he got the time off of the bill, he got his watch back and they left the store.