Sales Tax refund to customer after it has been filed

[Donna Pfeifer]

I’ve been looking for the proper way to handle this situation.
In November, I invoiced a customer for a job that included sales tax. At the end of the month, I paid it to the state as required.In mid December, they payed sans sales tax and sent an exemption certificate.
I showed the balance as paid with cash and then entered a refund for it from checking.
But now, when I need to pay sales tax, my sales tax account is off.
I have not reconciled December yet.
How should this be entered? 
Thanks in advance
Donna Pfeifer