Question

Someone came into my store and stole $100 from the register came back five minutes later with the same hundred dollars for $70 worth of items and I gave them back how much did I lose

Answer 1

$100
The thief came back with the same $100 for an item worth $70 , receiving a change of $30. In other words, it can be summarized like this:
$100=$70 + $30(change from the item bought).

Answer 2

$200
They first took $100 then came back and bought $70 worth of merchandise and paid with the 100 dollars they stole, in turn you gave them change of 30 dollars. So you are now short $130 in the register and 70 in merchandise for a total of $200.