**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


line = inv.group_lines(iml, line)
            journal = inv.journal_id.with_context(ctx)
            line = inv.finalize_invoice_move_lines(line)
            date = inv.date or date_invoice
            move_vals = {
                'ref': inv.reference,
                'line_ids': line,
                'journal_id': journal.id,
                'date': date,
                'narration': inv.comment,
            }
            ctx['company_id'] = inv.company_id.id
            ctx['dont_create_taxes'] = True
            ctx['invoice'] = inv
            ctx_nolang = ctx.copy()
            ctx_nolang.pop('lang', None)
            move = account_move.with_context(ctx_nolang).create(move_vals)
            
            
            move.post()
            
            vals = {
                'move_id': move.id,
                'date': date,
                'move_name': move.name,
            }
            inv.with_context(ctx).write(vals)
        return True
    @api.multi
    def invoice_validate(self):