**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


property_recs = self.env['ir.property'].search([('value_reference', 'in', ['account.payment.term,%s'%payment_term.id for payment_term in self])])
        property_recs.unlink()
        return super(AccountPaymentTerm, self).unlink()
class AccountPaymentTermLine(models.Model):
    _name = "account.payment.term.line"
    _description = "Payment Term Line"
    _order = "sequence, id"
    value = fields.Selection([
            ('balance', 'Balance'),
            ('percent', 'Percent'),
            ('fixed', 'Fixed Amount')
        ], string='Type', required=True, default='balance',
        help="Select here the kind of valuation related to this payment term line.")
    value_amount = fields.Float(string='Value', digits=dp.get_precision('Payment Term'), help="For percent enter a ratio between 0-100.")
    days = fields.Integer(string='Number of Days', required=True, default=0)
    option = fields.Selection([
            ('day_after_invoice_date', 'Day(s) after the invoice date'),
            ('fix_day_following_month', 'Day(s) after the end of the invoice month (Net EOM)'),
            ('last_day_following_month', 'Last day of following month'),
            ('last_day_current_month', 'Last day of current month'),
        ],
        default='day_after_invoice_date', required=True, string='Options'
        )
    payment_id = fields.Many2one('account.payment.term', string='Payment Term', required=True, index=True, ondelete='cascade')
    sequence = fields.Integer(default=10, help="Gives the sequence order when displaying a list of payment term lines.")
    @api.one
    @api.constrains('value', 'value_amount')
    def _check_percent(self):