Hi 

you can inherit the pos config and add the custom message that you needed in _reset_default_on_vals function.

class PosConfig(models.Model):
    _inherit = 'pos.config'

    def _reset_default_on_vals(self, vals):
        if 'tip_product_id' in vals and any(self.mapped('iface_tipproduct')) and not vals['tip_product_id']:
            default_product = self.env.ref('point_of_sale.product_product_tip', False)
            if default_product:
                vals['tip_product_id'] = default_product.id
            else:
                raise UserError(_('Add the message content that you needed'))

Regards