There are many real world complications to this problem. However, if we're just looking at the surface, he lost $100 worth of currency and goods.
Think about it this way: The owner had $100 in cash and $70 in goods. The man takes the 100 and leaves. Owner now has just the goods. Man comes back replaces the 100. He now grabs the goods ($70) and the change ($30) 70 + 30 = 100.
The owner lost $100 in both goods and currency.
REAL WORLD APPLICATION:
The owner would not have lost this much money. Most stores pay a wholesale price rather than paying per unit at retail value. The $70 worth of goods would not have cost the store $70 to purchase.
Example: Your local GameStop may have many copies of Madden for sale but, surely they did not pay $60 per Madden disc, the retail value for a new game. They paid one set price and were given a number of units to sell at their store for that price. When the math is done on it, you would see that they paid less than that per unit. Otherwise, they would have no incentive to be in business... in fact, there would be no business at all, they would just be a middle man for no reason; they'd make no money on the exchange.
So the real world answer would be no one would be able to know how much money the owner lost other than the owner and his accountant/financial advisor.
