[stooken]

A customer buys a widget from a vendor for $3. The cusomter gives a $10 bill. The vendor does not usually deal with cash, so he goes to the business next door to get change. The vendor then gives the customer the widget and $7 in change. After the customer leaves, the business next door determines that the $10 bill is counterfeit. Embarassed, the vendor gives the neighboring business a genuine $10 bill from his wallet. How much has the vendor lost in the transaction?