The eternal struggle between mathematicians and economists on full display! This seemingly simple problem is a beautiful trap for the brain. The store lost $100 (the stolen bill) plus $30 (the change given back) = $130, right? WRONG! That's the cognitive illusion at work.
The correct answer is $100. The thief stole $100 and received $30 in legitimate change after purchasing $70 worth of goods. The store lost the original $100 bill and $70 worth of merchandise, but received back the same $100 bill, making the net loss exactly $100.
This is why accountants drink heavily and why double-entry bookkeeping was invented. Conservation of money is harder than conservation of energy!