So, say i invoiced a customer with the incorrect sales tax item which ends up being more than what they should have been taxed. They have already paid the invoice and it was in months past, so we already paid the sales tax to the state and cannot just change the original invoice. 

Would i create a new invoice dated this month, exact same items and amounts, but with the proper sales tax item this time? Then create a credit memo showing all the same items and amounts, but using the tax rate i used on the original invoice. Apply credit to the new invoice, then the left over amount on the credit memo is what we owe to the customer and from there i can apply else where or reimburse. 

Correct?