Use a journal entry:
Debit: Tax payable
Credit: Miscellaneous income (or some other income account you choose or create)
Here is the reasoning: you charged the customer $100 and applied a tax code for, say, 6% sales tax. You received $106 from the customer, posting $100 to a sales income account and $6 to Tax payable. The amount in Tax payable is being held in trust (in your bank account) for the tax authority. By extending you a timely payment discount, the tax authority has effectively become your customer by forgiving part of the debt you owe them for tax you collected on their behalf. (The IRS considers forgiven debts as income.) The 1.2% discount has to go somewhere, because you are no longer holding it in trust. So you now have $0.072 additional income.
Another way to think of it, forgetting all the transactions that really occurred, is that you charged the customer $100.072 and applied a tax rate of 5.924% (with a little bit of rounding error).