account_voucher_comision/models/res_config_settings.py

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


class VoucherConfigSettings(models.TransientModel):
    _inherit = 'res.config.settings'

    voucher_comision_porcentaje = fields.Float(
        string='Porcentaje Comisión'
    )

    @api.model
    def get_default_voucher_comision_porcentaje(self, fields):
        param = self.env['ir.config_parameter'].get_param(
            'voucher_comision.porcentaje', '2'
        )
        return {
            'voucher_comision_porcentaje': float(param)
        }

    @api.multi
    def set_voucher_comision_porcentaje(self):
        self.env['ir.config_parameter'].set_param(
            'voucher_comision.porcentaje',
            self.voucher_comision_porcentaje
        )