Let’s break this down:
The man steals $100 – the store is down $100.
He then buys $70 worth of goods, using that same stolen $100.
The store gets its own $100 back.
But gives him $70 worth of goods and $30 in change.
Final Loss:
$70 in goods
$30 in cash
Total Loss:
$70 + $30 = $100