**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


addons/account/models/account_invoice.py
ref = self.invoice_id.number
        return {
            'name': self.name,
            'date': self.invoice_id.date_invoice,
            'account_id': self.account_analytic_id.id,
            'unit_amount': self.quantity,
            'amount': self.price_subtotal_signed,
            'product_id': self.product_id.id,
            'product_uom_id': self.uom_id.id,
            'general_account_id': self.account_id.id,
            'ref': ref,
        }
    @api.one
    @api.depends('price_unit', 'discount', 'invoice_line_tax_ids', 'quantity',
        'product_id', 'invoice_id.partner_id', 'invoice_id.currency_id', 'invoice_id.company_id',
        'invoice_id.date_invoice')
    def _compute_price(self):
        currency = self.invoice_id and self.invoice_id.currency_id or None
        price = self.price_unit * (1 - (self.discount or 0.0) / 100.0)
        taxes = False
        if self.invoice_line_tax_ids:
            taxes = self.invoice_line_tax_ids.compute_all(price, currency, self.quantity, product=self.product_id, partner=self.invoice_id.partner_id)
        self.price_subtotal = price_subtotal_signed = taxes['total_excluded'] if taxes else self.quantity * price
        if self.invoice_id.currency_id and self.invoice_id.company_id and self.invoice_id.currency_id != self.invoice_id.company_id.currency_id:
            price_subtotal_signed = self.invoice_id.currency_id.with_context(date=self.invoice_id.date_invoice).compute(price_subtotal_signed, self.invoice_id.company_id.currency_id)
        sign = self.invoice_id.type in ['in_refund', 'out_refund'] and -1 or 1
        self.price_subtotal_signed = price_subtotal_signed * sign
    @api.model
    def _default_account(self):