User sent an invoice to QuickBooks Online that had a retainer applied, but the retainer showed up as a negative line item on the invoice in QuickBooks Online.
This is the expected behavior. If you apply a retainer to an invoice during its creation or as a payment on the invoice and then send the invoice to QuickBooks Online, the retainer does not show up as a payment in QuickBook Online. Rather, it shows up as a negative line item on the invoice itself. This is because QuickBooks Online does not handle retainers like Core does. Check the Core Help for details. You may also want to check out how QuickBooks Online handles retainers: Record a retainer or deposit in QuickBooks Online.