Ok, so I haven't looked at any of the other comments yet. In purely fiat terms he has lost 4 shillings. He exchanged the fake note worth 0 shillings for a 10 shilling note (+10) and gave the customer 4 shillings in change (-4). So the current tally is at (+6). The shopkeeper then must pay back the tobacconist 10 shillings for the false note (-10). This leaves the shopkeeper at -4 shillings. If we account for the chocolate that was effectively stolen (-6) the shopkeeper has lost 10 shillings.