**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


_name = "account.invoice.tax"
    _description = "Invoice Tax"
    _order = 'sequence'
    def _compute_base_amount(self):
        tax_grouped = {}
        for invoice in self.mapped('invoice_id'):
            tax_grouped[invoice.id] = invoice.get_taxes_values()
        for tax in self:
            tax.base = 0.0
            if tax.tax_id:
                key = tax.tax_id.get_grouping_key({
                    'tax_id': tax.tax_id.id,
                    'account_id': tax.account_id.id,
                    'account_analytic_id': tax.account_analytic_id.id,
                })
                if tax.invoice_id and key in tax_grouped[tax.invoice_id.id]:
                    tax.base = tax_grouped[tax.invoice_id.id][key]['base']
                else:
                    _logger.warning('Tax Base Amount not computable probably due to a change in an underlying tax (%s).', tax.tax_id.name)
    invoice_id = fields.Many2one('account.invoice', string='Invoice', ondelete='cascade', index=True)
    name = fields.Char(string='Tax Description', required=True)
    tax_id = fields.Many2one('account.tax', string='Tax', ondelete='restrict')
    account_id = fields.Many2one('account.account', string='Tax Account', required=True, domain=[('deprecated', '=', False)])
    account_analytic_id = fields.Many2one('account.analytic.account', string='Analytic account')
    amount = fields.Monetary()
    manual = fields.Boolean(default=True)
    sequence = fields.Integer(help="Gives the sequence order when displaying a list of invoice tax.")
    company_id = fields.Many2one('res.company', string='Company', related='account_id.company_id', store=True, readonly=True)
    currency_id = fields.Many2one('res.currency', related='invoice_id.currency_id', store=True, readonly=True)