# http://codereview.tryton.org/5871002/

diff -r 46c9a4316548 sale.py
--- a/modules/sale/sale.py	Thu Dec 04 23:41:16 2014 +0100
+++ b/modules/sale/sale.py	Wed Dec 17 21:27:11 2014 +0100
@@ -15,6 +15,8 @@
 from trytond.pyson import If, Eval, Bool, PYSONEncoder, Id
 from trytond.transaction import Transaction
 from trytond.pool import Pool, PoolMeta
+from trytond.config import config
+DIGITS = config.getint('digits', 'unit_price_digits', 4)

 __all__ = ['Sale', 'SaleIgnoredInvoice', 'SaleRecreatedInvoice',
     'SaleLine', 'SaleLineTax', 'SaleLineIgnoredMove',
@@ -1029,7 +1031,7 @@
     product_uom_category = fields.Function(
         fields.Many2One('product.uom.category', 'Product Uom Category'),
         'on_change_with_product_uom_category')
-    unit_price = fields.Numeric('Unit Price', digits=(16, 4),
+    unit_price = fields.Numeric('Unit Price', digits=(16, DIGITS),
         states={
             'invisible': Eval('type') != 'line',
             'required': Eval('type') == 'line',
@@ -1047,6 +1049,8 @@
         domain=[('parent', '=', None), ['OR',
                 ('group', '=', None),
                 ('group.kind', 'in', ['sale', 'both'])],
+                ('company', '=',
+                    Eval('_parent_sale', {}).get('company', -1)),
             ],
         states={
             'invisible': Eval('type') != 'line',
@@ -1365,8 +1369,10 @@
             if old_invoice_line.type != 'line':
                 continue
             if old_invoice_line.id not in skip_ids:
+                sign = (1.0 if invoice_type == old_invoice_line.invoice_type
+                    else -1.0)
                 quantity -= Uom.compute_qty(old_invoice_line.unit,
-                    old_invoice_line.quantity, self.unit)
+                    sign * old_invoice_line.quantity, self.unit)

         rounding = self.unit.rounding if self.unit else 0.01
         invoice_line.quantity = Uom.round(quantity, rounding)