Welcome to the Community, @userkellie.
In QuickBooks Online (QBO), you can still make a credit memo to the sub-customer even if there's no invoice yet. You can then apply it later on to the invoice once created.
Once the invoice is created, here's how to apply the credit memo:
You also have the option to automatically apply credit memos to invoices. To learn more about this, I suggest checking out this article: Create and apply credit memos or delayed credits in QuickBooks Online.
Feel free to leave a comment below. I'm always here if you have further questions. Have an amazing rest of the day!