FIX

balance is debit - credit whereas on tax return you want to see what vat has to be paid so : VAT on sales (credit) - VAT on purchases (debit).

account_tax_balance/models/account_tax.py

@@ -45,7 +45,10 @@ class AccountTax(models.Model):
     def compute_balance(self, tax_or_base='tax'):
         self.ensure_one()
         move_lines = self.get_move_lines_domain(tax_or_base=tax_or_base)
-        total = sum([l.balance for l in move_lines])
+        # balance is debit - credit whereas on tax return you want to see what
+        # vat has to be paid so:
+        # VAT on sales (credit) - VAT on purchases (debit).
+        total = -sum([l.balance for l in move_lines])
         return total
 
     def get_balance_domain(self, state_list):

account_tax_balance/tests/test_account_tax_balance.py

@@ -46,5 +46,5 @@ class TestAccountTaxBalance(TransactionCase):
         # change the state of invoice to open by clicking Validate button
         invoice.signal_workflow('invoice_open')
 
-        self.assertEquals(tax.base_balance, -100)
-        self.assertEquals(tax.balance, -10)
+        self.assertEquals(tax.base_balance, 100)
+        self.assertEquals(tax.balance, 10)