**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


"""Overridable hashcode generation for invoice lines. Lines having the same hashcode
        will be grouped together if the journal has the 'group line' option. Of course a module
        can add fields to invoice lines that would need to be tested too before merging lines
        or not."""
        return "%s-%s-%s-%s-%s-%s" % (
            invoice_line['account_id'],
            invoice_line.get('tax_ids', 'False'),
            invoice_line.get('tax_line_id', 'False'),
            invoice_line.get('product_id', 'False'),
            invoice_line.get('analytic_account_id', 'False'),
            invoice_line.get('date_maturity', 'False'),
        )
    def group_lines(self, iml, line):