First principle: don't edit shipped sales orders and previously sent invoices.

Here are the steps you need to do:

1. Create a negative invoice with the same products, services, and fees -- just enter negative quantities and the dollar amounts will show negative. At this point the customer's invoicing is balanced out, but the payments aren't yet.

2. For the first invoice (the November one), enter in a payment, with a payment type of "credit".

3. Then, for the negative invoice you just created, enter in a negative payment for the invoice total, with a payment type of "credit".

4. Enter in an inventory adjustment if the returned goods are going back into inventory.

At this point your customer's financials are balanced -- according to the database, for this transaction they don't owe you and money and you don't owe them any money.

Not to complicate things, but if they had already paid the invoice prior to returning the items, you would skip step 2, and then once you did step three the customer would have a negative balance for which you could send them a check or apply to a future order.