**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


TYPE2REFUND = {
    'out_invoice': 'out_refund',        
    'in_invoice': 'in_refund',          
    'out_refund': 'out_invoice',        
    'in_refund': 'in_invoice',          
}
MAGIC_COLUMNS = ('id', 'create_uid', 'create_date', 'write_uid', 'write_date')
class AccountInvoice(models.Model):
    _name = "account.invoice"
    _inherit = ['mail.thread']
    _description = "Invoice"
    _order = "date_invoice desc, number desc, id desc"
    @api.one
    @api.depends('invoice_line_ids.price_subtotal', 'tax_line_ids.amount', 'currency_id', 'company_id', 'date_invoice', 'type')
    def _compute_amount(self):
        self.amount_untaxed = sum(line.price_subtotal for line in self.invoice_line_ids)
        self.amount_tax = sum(line.amount for line in self.tax_line_ids)
        self.amount_total = self.amount_untaxed + self.amount_tax
        amount_total_company_signed = self.amount_total
        amount_untaxed_signed = self.amount_untaxed
        if self.currency_id and self.company_id and self.currency_id != self.company_id.currency_id:
            currency_id = self.currency_id.with_context(date=self.date_invoice)
            amount_total_company_signed = currency_id.compute(self.amount_total, self.company_id.currency_id)
            amount_untaxed_signed = currency_id.compute(self.amount_untaxed, self.company_id.currency_id)
        sign = self.type in ['in_refund', 'out_refund'] and -1 or 1
        self.amount_total_company_signed = amount_total_company_signed * sign
        self.amount_total_signed = self.amount_total * sign
        self.amount_untaxed_signed = amount_untaxed_signed * sign
    @api.model
    def _default_journal(self):