diff -r 2570649c9bbb invoice.py
--- a/trytond/trytond/modules/account_invoice/invoice.py	Wed May 09 12:07:11 2018 +0200
+++ b/trytond/trytond/modules/account_invoice/invoice.py	Mon Aug 27 07:24:09 2018 +0200
@@ -2555,7 +2555,7 @@
             amount_second_currency = self.start.amount
             second_currency = self.start.currency
 
-        if amount > invoice.amount_to_pay and \
+        if self.start.amount > invoice.amount_to_pay and \
                 self.ask.type != 'writeoff':
             self.raise_user_error('amount_greater_amount_to_pay',
                 (invoice.rec_name,))