diff --git a/modules/purchase/purchase.py b/modules/purchase/purchase.py
index b9f1c5f337..be4462e1d5 100644
--- a/modules/purchase/purchase.py
+++ b/modules/purchase/purchase.py
@@ -1,6 +1,7 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 import datetime
+import math
 from itertools import chain
 from decimal import Decimal
 
@@ -32,6 +33,10 @@ from trytond.modules.product import price_digits, round_price
 from .exceptions import PurchaseQuotationError, PartyLocationError
 
 
+def samesign(a, b):
+    return math.copysign(a, b) == a
+
+
 def get_shipments_returns(model_name):
     "Computes the returns or shipments"
     def method(self, name):
@@ -1585,7 +1590,8 @@ class Line(sequence_ordered(), ModelSQL, ModelView):
                     gettext('purchase'
                         '.msg_purchase_missing_account_expense',
                         purchase=self.purchase.rec_name))
-        invoice_line.stock_moves = self._get_invoice_line_moves()
+        if samesign(self.quantity, invoice_line.quantity):
+            invoice_line.stock_moves = self._get_invoice_line_moves()
         return [invoice_line]
 
     def _get_invoice_not_line(self):
@@ -1713,9 +1719,11 @@ class Line(sequence_ordered(), ModelSQL, ModelView):
     def _get_move_invoice_lines(self, move_type):
         'Return the invoice lines that should be shipped'
         if self.purchase.invoice_method == 'order':
-            return [l for l in self.invoice_lines]
+            lines = self.invoice_lines
         else:
-            return [l for l in self.invoice_lines if not l.stock_moves]
+            lines = filter(lambda l: not l.stock_moves, self.invoice_lines)
+        return list(filter(
+                lambda l: samesign(self.quantity, l.quantity), lines))
 
     def set_actual_quantity(self):
         pool = Pool()
diff --git a/modules/sale/sale.py b/modules/sale/sale.py
index a774c93166..e3750d0997 100644
--- a/modules/sale/sale.py
+++ b/modules/sale/sale.py
@@ -1,6 +1,7 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 import datetime
+import math
 from decimal import Decimal
 from itertools import groupby, chain
 from functools import partial
@@ -29,6 +30,10 @@ from .exceptions import (
     SaleValidationError, SaleQuotationError, PartyLocationError)
 
 
+def samesign(a, b):
+    return math.copysign(a, b) == a
+
+
 def get_shipments_returns(model_name):
     "Computes the returns or shipments"
     def method(self, name):
@@ -1533,7 +1538,8 @@ class SaleLine(TaxableMixin, sequence_ordered(), ModelSQL, ModelView):
                 raise AccountError(
                     gettext('sale.msg_sale_missing_account_revenue',
                         sale=self.sale.rec_name))
-        invoice_line.stock_moves = self._get_invoice_line_moves()
+        if samesign(self.quantity, invoice_line.quantity):
+            invoice_line.stock_moves = self._get_invoice_line_moves()
         return [invoice_line]
 
     def _get_invoice_not_line(self):
@@ -1685,7 +1691,8 @@ class SaleLine(TaxableMixin, sequence_ordered(), ModelSQL, ModelView):
                         and invoice_line.invoice.state == 'paid'):
                     if invoice_line.moved_quantity < invoice_line.quantity:
                         invoice_lines.append(invoice_line)
-        return invoice_lines
+        return list(filter(
+                lambda l: samesign(self.quantity, l.quantity), invoice_lines))
 
     def set_actual_quantity(self):
         pool = Pool()