Customer Discounts

Greetings, @TerryS54. We are thrilled to have you join our Community space and appreciate your visit to our forum. Allow me to share insights into your concern about applying a cash discount when posting customer payments. 

In QuickBooks Online, you can only add a discount upon creating the sales transaction, either as a discount or as a value these will automatically deduct the total amount. Moreover, giving cash discounts when posting customer payments is not yet available in QuickBooks.

Furthermore, to be able to add a discount to payments you need to turn on the discount feature from the Accounts and Settings page so the discount option will show up on your transaction.

You can also check this article for more details about applying discounts:

Add a discount to an invoice or sales receipt in QuickBooks Online

Please don't hesitate to send a reply if there are any additional questions regarding adding a discount in QuickBooks Online. Have an awesome Day!