How to invoice for less than the full balance?

We are a non-profit and create invoices for our member's total annual donations (a "pledge"). Let's say someone pledges $1,600 to be paid in equal quarterly amounts of $400. They've already paid $400 and have an outstanding balance of $1,200. Come the next quarter, when I email them an invoice, I want the invoice to default to a payment amount of $400, not $1,200. But QB sets the default to the full balance of $1,200.  How do I get QB to invoice them for $400?

NOTE: I am well aware that the recipient can edit the payment amount to $400, that's not my question. My question is, can I get QB to invoice them for $400 instead of $12,000?

Example: see screenshots:

1. Invoice is $1,600

2. One payment of $400 was made

3. Outstanding balance is $1,200

4. When I invoice them, the payment screen defaults to $1,200. How can I get it to default to $400??  Do I have to create separate invoices? Is there another way to do it?