Use discount.

https://quickbooks.intuit.com/learn-support/en-us/service-items/add-a-discount-to-an-invoice-or-sale...

This is is an enormous QB fail that customers can’t get a line item on an invoice for a credit memo or even a separate emailed credit memo.  This is an awful workaround.  It creates another account to be managed.

This isn’t a new issue.  I’m sure you googled for solutions and found the frustration everywhere.  Intuit neither fixes this nor justifies their failure to act.  It’s mysterious why they would adopt such a customer hostile attitude.