Good to see you here in the Community, vivl2010.
QuickBooks Online lets you record a partial payment on an invoice. You can record the first payment that's due now by following the steps below.
You can follow the same steps when recording December's payment. I'm leaving you this write-up: Record invoice payments in QuickBooks Online. It contains information on how to record multiple payments in QuickBooks Online. Also, this will provide you steps on how to find customers' balances.
Please let me know if you have any other questions. I'm always here to help and provide you additional assistance. Have a wonderful day!