odoo-event-addons/event_sale_balance/models/event_registration.py

from odoo import models, api


class EventRegistration(models.Model):
    _inherit = "event.registration"

    def action_set_done(self):
        super().action_set_done()
        for regis in self:
            tic_prod = regis.event_ticket_id.product_id
            bal_prod = tic_prod.balance_variant_id
            if tic_prod.is_event_deposit and bal_prod:
                order = regis.sale_order_id
                lines = order.order_line
                line = lines.filtered(lambda sol: sol.product_id == bal_prod, limit=1)
                if line:
                    # Update existing line
                    update_values = order._prepare_order_line_update_values(
                        line, line.product_uom_qty + 1
                    )
                    if update_values:
                        order._update_cart_line_values(line, update_values)
                else:
                    # Create new line
                    line_values = order._prepare_order_line_values(bal_prod, 1)
                    line.sudo().create(line_values)