# Preventing Double Charges On Amended Transactions

There are certain circumstances that require a sales transaction to be amended by the merchant. When a [Capture](/apis/payments-platform-rest/openapi/transactions/transactionscapture) or [Sale/Purchase](/apis/payments-platform-rest/openapi/transactions/transactionssale) request is submitted, Shift4's Gateway searches the current batch for the invoice number specified. If the specified invoice is not found, a new transaction with the specified invoice number is created. For Food and Beverage (F&B) merchants, this may create potential double charge scenarios when a transaction has been marked as settled or captured, and a tip adjustment is made for the transaction after batch closing.

For example, an F&B merchant may send an Authorization for a total of $10 on Invoice 123, then a Capture half an hour later for $15 (including the tip) on Invoice 123.  Five hours later, the batch containing Invoice 123 is closed.  Afterward, an employee realizes that the tip was entered incorrectly, so they send a Capture for $13 on Invoice 123 intending to correct it.  However, since the batch containing the original invoice has already been closed, a new invoice is created and the customer is charged both the original $15 and the $13 from the second invoice, for a total of $28.

To avoid this, the INVMUSTEXIST API Option can be used when attempting to amend a transaction.  This option prevents a double charge from occurring by triggering an error when the original invoice is not found, instead of automatically creating a new invoice. Your interface should include this API Option for only F&B transactions using [Capture](/apis/payments-platform-rest/openapi/transactions/transactionscapture) or [Sale/Purchase](/apis/payments-platform-rest/openapi/transactions/transactionssale).