**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


"""Function used in other module to compute the taxes on a fresh invoice created (onchanges did not applied)"""
        account_invoice_tax = self.env['account.invoice.tax']
        ctx = dict(self._context)
        for invoice in self:
            
            self._cr.execute("DELETE FROM account_invoice_tax WHERE invoice_id=%s AND manual is False", (invoice.id,))
            self.invalidate_cache()
            
            tax_grouped = invoice.get_taxes_values()
            
            for tax in tax_grouped.values():
                account_invoice_tax.create(tax)
        
        return self.with_context(ctx).write({'invoice_line_ids': []})
    @api.multi
    def confirm_paid(self):
        return self.write({'state': 'paid'})
    @api.multi
    def unlink(self):
        for invoice in self:
            if invoice.state not in ('draft', 'cancel'):
                raise UserError(_('You cannot delete an invoice which is not draft or cancelled. You should refund it instead.'))
            elif invoice.move_name:
                raise UserError(_('You cannot delete an invoice after it has been validated (and received a number). You can set it back to "Draft" state and modify its content, then re-confirm it.'))
        return super(AccountInvoice, self).unlink()
    @api.onchange('invoice_line_ids')
    def _onchange_invoice_line_ids(self):