Hey there, sb-alehzayis.

I can help you reflect the discount in the  sales tax liability in QuickBooks Desktop. You can process a sales tax adjustment to reflect the transaction.

Here's how:

1. Go to the Vendors menu and select Sales Tax then Adjust Sales Tax Due.
2. In the Sales Tax Adjustment window:
   1. Enter the adjustment date, sales tax vendor, adjustment account, and other relevant information. Note: Do NOT use the sales tax payable for the adjustment account. Choose an Income account if you are making the adjustment because you received a discount/credit or if you are entering a negative rounding error. Choose an Expense account if you are making the adjustment because you need to add penalties and fines or if you are entering a positive rounding error.
   2. In the Adjustment section, choose if you need to increase or reduce sales tax then enter the adjustment amount.
   3. (Optional) In the Memo field, enter any additional note.
   4. Select OK.

To know more sales tax adjustment process, you can go through this article: Process sales tax adjustment.

Additionally, you can take a look at these pointers to help guide you more in managing sales tax in QuickBooks:

Show multiple sales tax items on an invoice

Pay sales tax

Feel free to drop me a reply below if you have any other sales tax concerns. I'll be more than willing to lend you a hand.