account_voucher_comision/models/account_voucher.py

# -*- coding: utf-8 -*-
from openerp import models, fields, api


class AccountVoucher(models.Model):
    _inherit = 'account.voucher'

    comision = fields.Float(
        string='Comisión',
        compute='_compute_comision',
        store=True
    )

    @api.depends('amount')
    def _compute_comision(self):

        param = self.env['ir.config_parameter'].get_param(
            'voucher_comision.porcentaje', '2'
        )

        porcentaje = float(param)

        for rec in self:
            rec.comision = rec.amount * porcentaje / 100 if rec.amount else 0