**Impacted versions:**
* 9.0

**Steps to reproduce:**
* Enable multi currencies
* Make sure that the current rate of a foreign currency (i.e. EUR) is not 1.0 (i.e. 0.84)
* Create a purchasable product with a cost (i.e. 33$)
* Create a vendor bill in a foreign currency (i.e. EUR)
* Add the product on the invoice

**Current behavior:**
* The unit price will be 23.28 == (33 * 0.84) * 0.84

**Expected behavior:**
* The unit price should be 27,72 == 33 * 0.84
* Probably due to this line of code: https://github.com/odoo/odoo/blob/9.0/addons/account/models/account_invoice.py#L1241

**Video/Screenshot link (optional):**
* On Runbot Community 9.0: https://www.youtube.com/watch?v=60qPmczo5vk


if self.value == 'percent' and (self.value_amount < 0.0 or self.value_amount > 100.0):
            raise UserError(_('Percentages for Payment Term Line must be between 0 and 100.'))
    @api.onchange('option')
    def _onchange_option(self):
        if self.option in ('last_day_current_month', 'last_day_following_month'):
            self.days = 0
class MailComposeMessage(models.Model):
    _inherit = 'mail.compose.message'
    @api.multi
    def send_mail(self, auto_commit=False):
        context = self._context
        if context.get('default_model') == 'account.invoice' and \
                context.get('default_res_id') and context.get('mark_invoice_as_sent'):
            invoice = self.env['account.invoice'].browse(context['default_res_id'])
            invoice = invoice.with_context(mail_post_autofollow=True)
            invoice.sent = True
            invoice.message_post(body=_("Invoice sent"))
        return super(MailComposeMessage, self).send_mail(auto_commit=auto_commit)
class ProductProduct(models.Model):