**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


amount_currency = company_currency.with_context(ctx).compute(t[1], inv.currency_id)
                    else:
                        amount_currency = False
                    
                    res_amount_currency -= amount_currency or 0
                    if i + 1 == len(totlines):
                        amount_currency += res_amount_currency
                    iml.append({
                        'type': 'dest',
                        'name': name,
                        'price': t[1],
                        'account_id': inv.account_id.id,
                        'date_maturity': t[0],
                        'amount_currency': diff_currency and amount_currency,
                        'currency_id': diff_currency and inv.currency_id.id,
                        'invoice_id': inv.id
                    })
            else:
                iml.append({
                    'type': 'dest',
                    'name': name,
                    'price': total,
                    'account_id': inv.account_id.id,
                    'date_maturity': inv.date_due,
                    'amount_currency': diff_currency and total_currency,
                    'currency_id': diff_currency and inv.currency_id.id,
                    'invoice_id': inv.id
                })
            part = self.env['res.partner']._find_accounting_partner(inv.partner_id)
            line = [(0, 0, self.line_get_convert(l, part.id)) for l in iml]