Copy sale_agreement_id from invoice to refund

agreement_account/models/account_invoice.py

@@ -3,7 +3,7 @@
 # @author: Alexis de Lattre <alexis.delattre@akretion.com>
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
-from odoo import models, fields
+from odoo import models, fields, api
 
 
 class AccountInvoice(models.Model):
@@ -13,3 +13,14 @@ class AccountInvoice(models.Model):
         'sale.agreement', string='Sale Agreement', ondelete='restrict',
         readonly=True, states={'draft': [('readonly', False)]},
         track_visibility='onchange')
+
+    @api.model
+    def _prepare_refund(
+            self, invoice, date_invoice=None, date=None, description=None,
+            journal_id=None):
+        values = super(AccountInvoice, self)._prepare_refund(
+            invoice, date_invoice=date_invoice, date=date,
+            description=description, journal_id=journal_id)
+        if invoice.sale_agreement_id:
+            values['sale_agreement_id'] = invoice.sale_agreement_id.id
+        return values