If the customer used an app-based mobile bank card to pay, like Monzo, Revolut, Starling etc. and the payment was not successful, the payment may initially still appear as having left the account.
Some banks do not have a pending transactions section but show all payments pending or confirmed in the same transaction list.
If the payment has not been taken, the record will either automatically update or disappear from the transactions list after a short period of time.
If this has not happened after a few days and the payment still remains, please contact your online bank directly.