The amount paid by the buyer is lower than the amount generated by the system.
In this case, the payer receives an e-mail with a link through which he can make a surcharge.
The Seller may also accept or refund a transaction underpaid at the Merchant Panel.
If you want to accept transactions you should search (Transactions → Browse → Set transaction status to "incorrect") you should select "details" and click the "accept" button.
However, if you want to return, you should go the path as before. When you have found a transaction, you must click "refund transaction"