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You can use a service item to account for a customer cashback. To do that, follow the steps below:
- Click the Gear icon and pic Products and services.
- Tap New then choose a Service item.
- Fill in all the needed information.
- For the income account, use a Discounts/Refund Given. You can also use other accounts that you want to assign to the item for tracking.

- Once done, hit Save and close.
Now, you can assign this item on invoices by adding a negative amount to show a reduction. See this:

Now, if you are the one who received a cash back or a refund for purchases made on your credit card, use the steps below:
What you should do:
- Click + New then choose Credit card credit.
- Find the appropriate account from the Bank/Credit Account drop-down box.
- Under the Category details section, select the account that you have for your cash back rewards whether expense/income account as you see fit.
Note: If you don't have an account to track your cash back rewards, you need to create one.
- Enter the other needed information.
- Select Save and close.
I'll be adding our Resource Guide here to help you with your tasks in QBO: QuickBooks Tutorials
If there are other things you want to ask or need help with, let me know in the comment below. I'll be right here to help you any time. Have a good one!