Hi! 

I'm trying to get a field for the sale order balance. We invoice a reservation in advance, and then before we give the service we ask for the second payment.

For the sake of not paying unnecesary taxes for unpayed invoices, we only invoice when we get the money.

I was trying to get a field with the balance of a sale, this is: 

Ammount_Total of the sale - Payed invoices. 

I tryed creating a new field in the debug. The field in the sale.order I was trying to do was: 

from odoo import models, fields, api

class SaleOrder(models.Model):

    _inherit = 'sale.order'

    balance = fields.Monetary(

        string='Balance',

        compute='_compute_balance',

        store=True,

        readonly=True,

        currency_field='currency_id',

        help="Balance of the sale order."

    )

    @api.depends('amount_total', 'invoice_ids.state', 'invoice_ids.amount_total')

    def _compute_balance(self):

        for order in self:

            paid_amount = sum(inv.amount_total for inv in order.invoice_ids.filtered(lambda inv: inv.state == 'paid'))

            order.balance = order.amount_total - paid_amount

Is there something I'm missing?

Thank you!