diff --git a/sale_project_fixed_price_task_completed_invoicing/models/project_task.py b/sale_project_fixed_price_task_completed_invoicing/models/project_task.py
index f9b4a5350ad..507bb047d6d 100644
--- a/sale_project_fixed_price_task_completed_invoicing/models/project_task.py
+++ b/sale_project_fixed_price_task_completed_invoicing/models/project_task.py
@@ -21,7 +21,9 @@ def toggle_invoiceable(self):
         for task in self:
             # We dont' want to modify when the related SOLine is invoiced
             if (not task.sale_line_id or
-                    task.sale_line_id.state in ('done', 'cancel')):
+                    task.sale_line_id.state in ('done', 'cancel') or
+                    task.sale_line_id.invoice_status in ('to invoice',
+                                                         'invoiced')):
                 raise UserError(_("You cannot modify the status if there is "
                                   "no Sale Order Line or if it has been "
                                   "invoiced."))
@@ -38,6 +40,7 @@ def write(self, vals):
                                       )
         return super(ProjectTask, self).write(vals)
 
+    @api.model
     def create(self, vals):
         SOLine = self.env['sale.order.line']
         so_line = SOLine.browse(vals.get('sale_line_id'))
diff --git a/sale_project_fixed_price_task_completed_invoicing/models/sale_order_line.py b/sale_project_fixed_price_task_completed_invoicing/models/sale_order_line.py
index abfeac6b24d..bc57524fc25 100644
--- a/sale_project_fixed_price_task_completed_invoicing/models/sale_order_line.py
+++ b/sale_project_fixed_price_task_completed_invoicing/models/sale_order_line.py
@@ -15,6 +15,7 @@ def create(self, vals):
         if (line.state == 'sale' and not line.order_id.project_id and
                 line.product_id.track_service in ('completed_task', )):
             line.order_id._create_analytic_account()
+        return line
 
     @api.constrains('product_id')
     def _onchange_product_id(self):
diff --git a/sale_project_fixed_price_task_completed_invoicing/tests/__init__.py b/sale_project_fixed_price_task_completed_invoicing/tests/__init__.py
index e69de29bb2d..6524ec69da9 100644
--- a/sale_project_fixed_price_task_completed_invoicing/tests/__init__.py
+++ b/sale_project_fixed_price_task_completed_invoicing/tests/__init__.py
@@ -0,0 +1,2 @@
+# -*- coding: utf-8 -*-
+from . import test_sale_project_fixed_price_task_completed_invoicing
diff --git a/sale_project_fixed_price_task_completed_invoicing/tests/test_sale_project_fixed_price_task_completed_invoicing.py b/sale_project_fixed_price_task_completed_invoicing/tests/test_sale_project_fixed_price_task_completed_invoicing.py
new file mode 100644
index 00000000000..f46d5ca931c
--- /dev/null
+++ b/sale_project_fixed_price_task_completed_invoicing/tests/test_sale_project_fixed_price_task_completed_invoicing.py
@@ -0,0 +1,56 @@
+# -*- coding: utf-8 -*-
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
+
+from odoo.addons.sale.tests.test_sale_common import TestSale
+from odoo.exceptions import UserError  # , ValidationError
+
+
+class TestSaleProjectFixedPrice(TestSale):
+
+    def test_sale_project_fixed_price(self):
+        # The product comes from 'sale_timesheet' the only modification to add
+        # is for the track_service
+        prod_task = self.env.ref('product.product_product_1')
+        prd_vals = {
+            'track_service': 'completed_task',
+        }
+        prod_task.write(prd_vals)
+        so_vals = {
+            'partner_id': self.partner.id,
+            'partner_invoice_id': self.partner.id,
+            'partner_shipping_id': self.partner.id,
+            'order_line': [(0, 0, {'name': prod_task.name,
+                                   'product_id': prod_task.id,
+                                   'product_uom_qty': 1,
+                                   'product_uom': prod_task.uom_id.id,
+                                   'price_unit': prod_task.list_price})],
+            'pricelist_id': self.env.ref('product.list0').id,
+        }
+        so = self.env['sale.order'].create(so_vals)
+        so.action_confirm()
+
+        # check task creation
+        project = self.env.ref('sale_timesheet.project_GAP')
+        task = project.task_ids.filtered(
+            lambda t: t.name == '%s:%s' % (so.name, prod_task.name))
+        self.assertTrue(task, 'Sale Service: task is not created')
+        self.assertEqual(task.partner_id, so.partner_id,
+                         'Sale Service: customer should be the same on task '
+                         'and on SO')
+        self.assertTrue(task.fixed_price)
+
+        # check Task validation. It should update the delivered quantity
+        line = so.order_line
+        self.assertFalse(line.product_uom_qty == line.qty_delivered,
+                         'Sale Service: line should be invoiced completely')
+        task.toggle_invoiceable()
+        self.assertTrue(task.invoiceable, 'The task should be invoiceable')
+        self.assertTrue(line.product_uom_qty == line.qty_delivered,
+                        'Sale Service: line should be invoiced completely')
+
+        # Impossible to change task invoicable after validation of soline
+        so.action_invoice_create()
+        self.assertEqual(so.invoice_status,
+                         'invoiced', 'SO should be invoiced')
+        with self.assertRaises(UserError):
+            task.toggle_invoiceable()