restrict_invoice_credits/models/account_invoice.py

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

class account_invoice(models.Model):
    _inherit = 'account.invoice'
    _name = 'account.invoice'

    enable_credit = fields.Boolean('Habilitar Credito')

    _defaults = {
        'enable_credit': False,
    }

    @api.one
    @api.onchange('credito')
    def cambiar_estado_credito(self):
        self.contado = not self.credito

    @api.one
    @api.onchange('contado')
    def cambiar_estado_contado(self):
        if self.enable_credit == False:
            self.contado =True
        else:
            self.credito = not self.contado

    @api.one
    def habilitar_check(self):
        invoice = self.env['account.invoice'].search([('id', '=', self.id)])
        if invoice:
            if invoice.enable_credit == False:
                invoice.write({'enable_credit' : True})
            else:
                invoice.write({'enable_credit' : False, 'credito' : False, 'contado' : True })