diff --git a/addons/account/models/account_invoice.py b/addons/account/models/account_invoice.py
index 5b09af3be43..f653941e7b6 100644
--- a/addons/account/models/account_invoice.py
+++ b/addons/account/models/account_invoice.py
@@ -985,6 +985,7 @@ class AccountInvoice(models.Model):
         values['state'] = 'draft'
         values['number'] = False
         values['origin'] = invoice.number
+        values['payment_term_id'] = False
 
         if date:
             values['date'] = date
@@ -1365,6 +1366,7 @@ class AccountPaymentTerm(models.Model):
     def compute(self, value, date_ref=False):
         date_ref = date_ref or fields.Date.today()
         amount = value
+        sign = value < 0 and -1 or 1
         result = []
         if self.env.context.get('currency_id'):
             currency = self.env['res.currency'].browse(self.env.context['currency_id'])
@@ -1373,7 +1375,7 @@ class AccountPaymentTerm(models.Model):
         prec = currency.decimal_places
         for line in self.line_ids:
             if line.value == 'fixed':
-                amt = round(line.value_amount, prec)
+                amt = sign * round(line.value_amount, prec)
             elif line.value == 'percent':
                 amt = round(value * (line.value_amount / 100.0), prec)
             elif line.value == 'balance':