What should we do for e-Invoice if a customer underpays or overpays?

If the original Sales Invoice (e-Invoice) amount for the transaction is correct, you do not need to submit any adjustment-type e-Invoice (such as a Debit Note e-Invoice, Credit Note e-Invoice, or Refund Note e-Invoice).

Simply record the underpayment or overpayment in your system as usual:

 - Received Payment → for recording payments received.

 - Customer Refund → for refunding any overpayment to the customer.

For this type of adjustment, no e-Invoice is required because the final amount on the Sales Invoice e-Invoice remains unchanged.

Recommendation: Keep payment records and any correspondence with the customer to support the difference between the invoiced amount and the actual payment.