diff --git a/addons/sale/models/sale.py b/addons/sale/models/sale.py
index 4ff43dd1b35..c87649c8166 100644
--- a/addons/sale/models/sale.py
+++ b/addons/sale/models/sale.py
@@ -743,32 +743,34 @@ class SaleOrderLine(models.Model):
 
         return line
 
+    def _update_line_quantity(self, values):
+        orders = self.mapped('order_id')
+        for order in orders:
+            order_lines = self.filtered(lambda x: x.order_id == order)
+            msg = ""
+            if any([values['product_uom_qty'] < x.product_uom_qty for x in order_lines]):
+                msg += "<b>" + _(
+                    'The ordered quantity has been decreased. Do not forget to take it into account on your invoices and delivery orders.') + '</b>'
+            msg += "<ul>"
+            for line in order_lines:
+                msg += "<li> %s:" % (line.product_id.display_name,)
+                msg += "<br/>" + _("Ordered Quantity") + ": %s -> %s <br/>" % (
+                line.product_uom_qty, float(values['product_uom_qty']),)
+                if line.product_id.type in ('consu', 'product'):
+                    msg += _("Delivered Quantity") + ": %s <br/>" % (line.qty_delivered,)
+                msg += _("Invoiced Quantity") + ": %s <br/>" % (line.qty_invoiced,)
+            msg += "</ul>"
+            order.message_post(body=msg)
+
     @api.multi
     def write(self, values):
         lines = False
-        changed_lines = False
         if 'product_uom_qty' in values:
             precision = self.env['decimal.precision'].precision_get('Product Unit of Measure')
             lines = self.filtered(
                 lambda r: r.state == 'sale' and float_compare(r.product_uom_qty, values['product_uom_qty'], precision_digits=precision) == -1)
-            changed_lines = self.filtered(
-                lambda r: r.state == 'sale' and float_compare(r.product_uom_qty, values['product_uom_qty'], precision_digits=precision) != 0)
-            if changed_lines:
-                orders = self.mapped('order_id')
-                for order in orders:
-                    order_lines = changed_lines.filtered(lambda x: x.order_id == order)
-                    msg = ""
-                    if any([values['product_uom_qty'] < x.product_uom_qty for x in order_lines]):
-                        msg += "<b>" + _('The ordered quantity has been decreased. Do not forget to take it into account on your invoices and delivery orders.') + '</b>'
-                    msg += "<ul>"
-                    for line in order_lines:
-                        msg += "<li> %s:" % (line.product_id.display_name,)
-                        msg += "<br/>" + _("Ordered Quantity") + ": %s -> %s <br/>" % (line.product_uom_qty, float(values['product_uom_qty']),)
-                        if line.product_id.type in ('consu', 'product'):
-                            msg += _("Delivered Quantity") + ": %s <br/>" % (line.qty_delivered,)
-                        msg += _("Invoiced Quantity") + ": %s <br/>" % (line.qty_invoiced,)
-                    msg += "</ul>"
-                    order.message_post(body=msg)
+            self.filtered(
+                lambda r: r.state == 'sale' and float_compare(r.product_uom_qty, values['product_uom_qty'], precision_digits=precision) != 0)._update_line_quantity(values)
         result = super(SaleOrderLine, self).write(values)
         if lines:
             lines._action_procurement_create()
diff --git a/addons/sale_stock/models/sale_order.py b/addons/sale_stock/models/sale_order.py
index 98d3aef829b..556c755cfe5 100644
--- a/addons/sale_stock/models/sale_order.py
+++ b/addons/sale_stock/models/sale_order.py
@@ -158,6 +158,10 @@ class SaleOrderLine(models.Model):
     @api.onchange('product_uom_qty')
     def _onchange_product_uom_qty(self):
         if self.state == 'sale' and self.product_id.type in ['product', 'consu'] and self.product_uom_qty < self._origin.product_uom_qty:
+            # Do not display this warning if the new quantity is below the delivered
+            # one; the `write` will raise an `UserError` anyway.
+            if self.product_uom_qty < self.qty_delivered:
+                return {}
             warning_mess = {
                 'title': _('Ordered quantity decreased!'),
                 'message' : _('You are decreasing the ordered quantity! Do not forget to manually update the delivery order if needed.'),
@@ -242,3 +246,13 @@ class SaleOrderLine(models.Model):
                     break
 
         return is_available
+
+    def _update_line_quantity(self, values):
+        if self.mapped('qty_delivered') and values['product_uom_qty'] < max(self.mapped('qty_delivered')):
+            raise UserError('You cannot decrease the ordered quantity below your delivered quantity.\n'
+                            'Create a return first.')
+        for line in self:
+            pickings = self.order_id.picking_ids.filtered(lambda p: p.state not in ('done', 'cancel'))
+            pickings.message_post("The quantity of %s has been updated from %d to %d in %s" %
+                                  (line.product_id.name, line.product_uom_qty, values['product_uom_qty'], self.order_id.name))
+        super(SaleOrderLine, self)._update_line_quantity(values)