Does the customer pay the tax ?
Do you receive a tax credit for the payment ?
(can you use the tax payment to offset other tax liabilities)
@Brucanna has already asked the important questions for understanding how to treat this tax. (I am responding only because you addressed your post to me.)
To amplify on the first question, though, if the customer pays this tax, how is that payment reported and to whom? And what confirmation of the payment, if any, do you receive, and from whom? And if the customer does not pay the tax, why are you deducting it from the balance due on the sales invoice?
Also, depending on your answers to all these questions, it is unlikely any journal entries will be involved. Very few transactions in Manager are entered that way.