Hi there, @julie95.
You'll want to add another line item on the invoice and enter the negative discounted amount. I've attached a screenshot below for your reference. 
Here's an article you can read for future reference. This can guide you through the detailed steps in how to add older transactions to your QuickBooks Self-Employed (QBSE) account.
Furthermore, I encourage checking our Help articles page. There, you can read some of our great articles that can guide you in managing your QuickBokks Self-Employed (QBSE) account efficiently.
Please let me know if there's anything else you need help with. I'll be around to assist you. Keep safe!