Which action must be performed to void a transaction after it has been completed?

To void a transaction that has already been completed, scanning the item to be voided is the appropriate action. This process typically requires the cashier or supervisor to retrieve the specific item in the system, allowing the system to recognize which transaction needs to be reversed. Scanning the item ensures that the system accurately processes the void, reflecting the correct changes in inventory and sales figures.

The other options don’t align with the standard procedures for voiding a transaction. Overriding the membership card or canceling it deals with membership issues rather than transaction voids, and re-entering the transaction amount does not effectively address the need to remove an item from a completed sale. The selected action aligns directly with the expected practices in retail transaction management.