contract/agreement_sale_condition_base/models/account_invoice.py

# Copyright 2019 Camptocamp SA
# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
from odoo import fields, models


class AccountInvoice(models.Model):
    _inherit = 'account.invoice'

    agreement_id = fields.Many2one('agreement', readonly=True)