**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 journal_id:
            journal = self.env['account.journal'].browse(journal_id)
        elif invoice['type'] == 'in_invoice':
            journal = self.env['account.journal'].search([('type', '=', 'purchase')], limit=1)
        else:
            journal = self.env['account.journal'].search([('type', '=', 'sale')], limit=1)
        values['journal_id'] = journal.id
        values['type'] = TYPE2REFUND[invoice['type']]
        values['date_invoice'] = date_invoice or fields.Date.context_today(invoice)
        values['state'] = 'draft'
        values['number'] = False
        values['origin'] = invoice.number
        values['payment_term_id'] = False
        if date:
            values['date'] = date
        if description:
            values['name'] = description
        return values
    @api.multi
    @api.returns('self')
    def refund(self, date_invoice=None, date=None, description=None, journal_id=None):
        new_invoices = self.browse()
        for invoice in self:
            
            values = self._prepare_refund(invoice, date_invoice=date_invoice, date=date,
                                    description=description, journal_id=journal_id)
            new_invoices += self.create(values)
        return new_invoices
    @api.v8
    def pay_and_reconcile(self, pay_journal, pay_amount=None, date=None, writeoff_acc=None):