This module allows to add comision en voucher.

models/account_voucher.py

@@ -11,7 +11,7 @@ class AccountVoucher(models.Model):
         store=True
     )
 
-    @api.depends('amount')
+    @api.depends('amount', 'state')
     def _compute_comision(self):
 
         param = self.env['ir.config_parameter'].get_param(
@@ -21,7 +21,7 @@ class AccountVoucher(models.Model):
         porcentaje = float(param)
 
         for rec in self:
-            if rec.amount:
+            if rec.state == 'posted' and rec.amount:
                 valor = rec.amount * porcentaje / 100
                 rec.comision = int(round(valor))
             else: