Final changes have the total discount field and translate

locale/es.po

@@ -37,3 +37,7 @@ msgstr "Bar"
 msgctxt "model:ir.model.button,string:sale_print_bill_button"
 msgid "Bill"
 msgstr "Subtotal"
+
+msgctxt "field:sale.sale,total_discount:"
+msgid "Total Discount"
+msgstr "Total Descuento"

sale.py

@@ -1,6 +1,8 @@
 from trytond.pool import Pool, PoolMeta
 from trytond.model import ModelView, fields
 from trytond.transaction import Transaction
+from trytond.modules.currency.fields import Monetary
+from decimal import Decimal
 
 import requests
 import json

view/sale_form.xml

@@ -12,7 +12,7 @@ this repository contains the full copyright notices and license terms. -->
 	</group>
 	<newline/>
     </xpath>
-    <xpath expr="/form/notebook/page[@id='purchase']/group[@id='amount']/field[@name='tax_amount']" position="after">
+    <xpath expr="/form/notebook/page[@id='sale']/group[@id='amount']/field[@name='tax_amount']" position="after">
 		<label name="total_discount"/>
 		<field name="total_discount"/>
     </xpath>