Hi, Jesse. Did your customer overpay their invoice?
QuickBooks Online automatically issues credits when a customer's payment exceeds the total of their invoice. These credits can be applied to your future invoices.

If no overpayment exists, it’s possible that a credit was added manually. You can verify this by going to the Gear icon and selecting the Audit log. This report shows exactly who created the transaction. If you find it was entered in error, delete the credit to correct your records.
To remove the entry, open the Credit Memo from the customer's profile. Once open, select More at the bottom of the screen, choose Delete, and then click Delete again to confirm.
Please respond to this post if you have additional information.