Thanks for posting your concern here in the Community. I can share some information about discount terms for expenses in QuickBooks Online (QBO).
You can set up a discount term when creating a bill. However, this won't automatically calculate in QBO. As a workaround, you can manually add a new line indicating it as a discount. Make sure that the amount has a negative sign (-) to subtract the amount from the original price.
I attached a sample screenshot for your reference.
While this option isn't available yet, I'd highly suggest submitting feedback and suggestions regarding this directly to our product developers for consideration in future product updates. You can submit this from within QBO account by selecting the Gear icon and Feedback or by going to the following link: QuickBooks Online Feature Requests. I'm going to submit feedback from my side as well.
That's it! Please let me know how it goes or if you're referring to something else. I'd be glad to answer should you have follow-up concerns. You take care.