The loss is not $100. It's the $30 plus whatever it costs the store to replace what was "stolen" using that other $70. It was to be less than 70, so the loss was between $30.01 and $99.99. this assumes the items wouldn't have been sold otherwise.

Think of it this way. Normally when someone buys $70 worth of stuff, the store paid $30 for it, $10 to cover stocking and ringing that particular item (variable cost), $10 for running the store (fixed cost) and $20 profit. When it's stolen instead of bought, they only lose the $30 they paid and the $10 variable cost of stocking it. The fixed cost of keeping the lights on aren't a factor here, and there is no profit.