1. Create a product called down payment.
Product type: Service and the Income account is Current liability.

2. By the time we are going to issue an invoice, create the credit note first.

2.1 Credit note invoice line:

2.2 Credit note journal items:

3. Create the invoice. Add the 'credit note' to reduce the account receivable.
3.1 Invoice customer is the same as #2 credit note customer. Also we can see 'outstanding payment' to apply to this customers based on #2 credit note.

3.2 'Add' the outstanding credit $500 and apply to the $1500 invoice. This reduce the $1500 invoice to $1000 with the 'partial' banner.