account_move.py

from odoo import _, api, fields, models
from odoo.exceptions import UserError


class AccountMove(models.Model):
    _inherit = "account.move"

    move_currency_id = fields.Many2one(
        "res.currency",
        "Secondary Currency",
        readonly=True,
        help="If you set a currency here, then this invoice values will be also stored in the related Account Move Secondary Currency",
    )

    move_inverse_currency_rate = fields.Float(
        digits=(16, 4), string="Account Move Secondary Currency Rate", readonly=True
    )

    @api.onchange("move_currency_id")
    def change_move_currency(self):
        if not self.move_currency_id:
            self.move_inverse_currency_rate = False
        else:
            self.move_inverse_currency_rate = self.move_currency_id._convert(
                1.0, self.company_id.currency_id, self.company_id, self.invoice_date or fields.Date.context_today(self)
            )

    @api.constrains("move_currency_id", "currency_id")
    def check_move_currency(self):
        for rec in self.filtered("move_currency_id"):
            if rec.move_currency_id == rec.currency_id:
                raise UserError(_("Secondary currency can not be the same as Invoice Currency"))
            if rec.currency_id != rec.company_id.currency_id:
                raise UserError(
                    _("Can not use Secondary currency if invoice is in a Currency different from Company Currency")
                )