The shopkeeper loses 10 shillings. he gives the 10 shilling note to the tobacconist and the tobacconist gives him lets say 10 single shillings. then he gives the four shillings change to the customer. since the customer gave him the fake bill the tobacconist wants his money back, so the sweetshop man gives him the six shillings the guy paid with plus four more to make it to 10 shillings. If he didn't have to pay the ten shillings meaning the tobacconist didn't find out he wouldn't have lost anything,because he made a trade with the customer. if it was an even trade than the cost of chocolate was exactly 6 shillings assuming the sweetshop owner didn't make a profit. I originally thought 14, but I'm sure it is 10 because he didn't lose the money that he gave to the customer, he traded him.