**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


currency = self.env.user.company_id.currency_id
        prec = currency.decimal_places
        for line in self.line_ids:
            if line.value == 'fixed':
                amt = sign * round(line.value_amount, prec)
            elif line.value == 'percent':
                amt = round(value * (line.value_amount / 100.0), prec)
            elif line.value == 'balance':
                amt = round(amount, prec)
            if amt:
                next_date = fields.Date.from_string(date_ref)
                if line.option == 'day_after_invoice_date':
                    next_date += relativedelta(days=line.days)
                elif line.option == 'fix_day_following_month':
                    next_first_date = next_date + relativedelta(day=1, months=1)  
                    next_date = next_first_date + relativedelta(days=line.days - 1)
                elif line.option == 'last_day_following_month':
                    next_date += relativedelta(day=31, months=1)  
                elif line.option == 'last_day_current_month':
                    next_date += relativedelta(day=31, months=0)  
                result.append((fields.Date.to_string(next_date), amt))
                amount -= amt
        amount = reduce(lambda x, y: x + y[1], result, 0.0)
        dist = round(value - amount, prec)
        if dist:
            last_date = result and result[-1][0] or fields.Date.today()
            result.append((last_date, dist))
        return result
    @api.multi
    def unlink(self):