I think if a groupon is unused then the merchant is not paid, so Groupon keeps the $10.
This is how kgbdeals works. However Groupon redeem expired vouchers (if asked) whereas kgbdeals refuse to redeem vouchers even if valid.
Groupon always allows you to redeem an expired voucher for the purchased value (e.g., $10 for $20, you can always get your $10 in credit from the merchant). I think the merchant must be able to keep the purchased portion of it or they'd be out a lot of cash over time if their Groupon was a particularly popular one.