I can share some insights on how you can use the credit and apply it to the invoice, @Koizumi008.

For this, you can create a custom credit service item so you can quickly add it to credit notes as a single line item. This way, it is different to the ones on the invoice. If your customer has returned inventory and you wish to update your quantity on hand, that's the only time you can input the product as a line item within the credit note.

Here are the steps you can follow to apply it:

1. Select + New.
2. Select Credit note.
3. In the Customer dropdown, select the customer's name.
4. Enter the credit note details, such as the date and the amount. Tip: 
5. When you're done, select Save and close.

See this article for the complete guide: Create and apply credit notes or delayed credits in QuickBooks Online

Please come back here if you need further help with applying a sales credit. I'll be more than happy to help you out. All the best