Maybe you can use the field Journal Items/Is Downpayment:

It relies on the first line being a downpayment, which should always be true

Hi,

Odoo does not separate Down Payment invoices and Regular invoices by default in the customer invoice list, but you can categorize them easily using an existing field. Every invoice automatically creates journal items, and inside account.move.line there is a boolean field named is_downpayment. You can use this field to build a reliable filter without any customization in the invoicing logic.

To make this useful, simply create a custom filter (saved search) in Accounting → Customers → Invoices or directly in Journal Items. Configure the filter so that "Journal Item → is_downpayment = True", which will instantly show all invoices that include a down payment line. You can also save another filter for is_downpayment = False to view only regular invoices. This gives you a quick, one-click way to distinguish between down payment invoices and regular invoices using a field Odoo already maintains.

Journal Item

Journal Entry

Regards