I see a lot of answers here that rely on assumptions about the storekeeper’s profit margin and a critical, but generally unstated assumption that the storekeeper could have sold his inventory only to the thief.
Reverse the order of the events. Someone buys the merchandise for $40 on Saturday… maybe it’s the thief, maybe it's some other customer. The identity of the customer doesn't affect the storekeeper’s profit. The cash from that sale is now part of the storekeeper’s wealth.
Then on Sunday, the thief steals $50 from the cash drawer… Clearly, the storekeeper is $50 poorer.