invoiceninja/app/Http/Requests/ClientPortal/ShowInvoiceRequest.php

<?php
/**
 * Invoice Ninja (https://invoiceninja.com).
 *
 * @link https://github.com/invoiceninja/invoiceninja source repository
 *
 * @copyright Copyright (c) 2020. Invoice Ninja LLC (https://invoiceninja.com)
 *
 * @license https://opensource.org/licenses/AAL
 */

namespace App\Http\Requests\ClientPortal;

use App\Http\Requests\Request;

class ShowInvoiceRequest extends Request
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize() : bool
    {
        return auth('contact')->user()->client->id == $this->invoice->client_id;
    }
}