United revenue management does audits to make sure each ticket is paid for. I would deduct the cost of the tickets from the debit account balance for the next couple of months so you don't wind up overdrawn. Eventually they will catch up to you and charge your card.

On an episode of Airline on A & E a similar thing happened to a passenger and they were caught at the gate. The gate agent insisted that the computer was showing the ticket was not paid for, while the passenger insisted their credit card was charged. The WN gate agent told the passenger they had to buy a new ticket at the walk up price in order to fly.

Good luck!

RIP...