first of all, Thank you to take the time to read this.

I am making a Payment Test app, with Java and Spring boot, but to take the ad data I am making a form with the data of that ad in my HTML template of the ad in which the user will click on the pay button .

This is the HTML so you can understand it better.

<meta charset="UTF-8">

<meta name="viewport" content="width=device-width, initial-scale=1.0">

<title>Detalle del Anuncio</title>

<h1 th:text="${anuncio.titulo}">Título del Anuncio</h1>

<img th:src="@{${anuncio.imageUrl}}" alt="Imagen del anuncio">

<p>Categoría: <span th:text="${anuncio.categoria}"></span></p>

<p>Estado: <span th:text="${anuncio.estado}"></span></p>

<p>Precio: <span th:text="${anuncio.precio}"></span></p>

<p>Descripción: <span th:text="${anuncio.descripcion}"></span></p>

<button id="btn">Comprar</button>

<input type="text" name="amount" th:value="${anuncio.precio}">

<input type="text" name="descripcion" th:value="${anuncio.titulo}">

<button type="submit">comprar</button>

public String pay(@PathVariable("id") Long id, @RequestParam("amount") Long amount, @RequestParam("descripcion")String description) {

    

    Anuncio anuncio = anuncioService.findAdById(id);

    if(anuncio == null) {

        //THIS IS A EXAMPLE ETROR HANDLING. ITS JUST A EXAMPLE

        

        //SOLO ES UN EJEMPLO SOBRE EL MANEJO DE ERRORES, ESTO ES AUN UNA PRUEBA.

        

        return "redirect:/nullAnonce";

    } else {

        Long originalPrice = anuncio.getPrecio(); 

        //JUST A EXAMPLE TAKING ONLY THE PRICE, IN A REAL CASE I WILL BE TAKING ALL.

        

        //ES SOLO UN EJEMPLO TOMANDO SOLO EL PRECIO, EN UN CASO REAL TOMARIAMOS NO SOLO EL PRECIO TAMBIEN LA DESCRIPCION Y ESAS COSAS. 

        if(amount != originalPrice) {

                        //JUST A EXAMPLE TAKING ONLY THE PRICE, IN A REAL CASE I WILL BE TAKING ALL.

            return "redirect:/PriceNotCorrect";

        } else {

            try {

        // Configurar las credenciales de PayPal

        String clientId = "CLIENTID";

        String clientSecret = "CLIENTSECRET";

        String mode = "sandbox"; // O "live" para producción



        // Crear el objeto APIContext

        APIContext apiContext = new APIContext(clientId, clientSecret, mode);



        // Configurar la cantidad y la transacción

        Amount paymentAmount = new Amount();

        paymentAmount.setCurrency("USD");

        paymentAmount.setTotal("500");



        Transaction transaction = new Transaction();

        transaction.setDescription("Descripción del pago");

        transaction.setAmount(paymentAmount);



        List<Transaction> transactions = new ArrayList<>();

        transactions.add(transaction);



        // Configurar el pagador

        Payer payer = new Payer();

        payer.setPaymentMethod("paypal");



        // Configurar el pago

        Payment payment = new Payment();

        payment.setIntent("sale");

        payment.setPayer(payer);

        payment.setTransactions(transactions);



        // Configurar las URLs de redirección

        RedirectUrls redirectUrls = new RedirectUrls();

        redirectUrls.setCancelUrl("https://localhost:3000/cancel");

        redirectUrls.setReturnUrl("https://localhost:3000/rooms");

        payment.setRedirectUrls(redirectUrls);



        // Crear el pago

        Payment createdPayment = payment.create(apiContext);



        // Obtener la URL de aprobación y redirigir al usuario

        String approvalUrl = createdPayment.getLinks().stream()

                .filter(link -> "approval_url".equals(link.getRel()))

                .findFirst()

                .orElseThrow(() -> new PayPalRESTException("Approval URL not found"))

                .getHref();



        return "redirect:" + approvalUrl;

    } catch (PayPalRESTException e) {

        e.printStackTrace();

        return "redirect:/error";

    }

        }

    }

    

}`

Sorry if it seems a bit ‘stupid’…

What we are trying to do is that even if a user changes the value to the price, it will be compared with the price in the database, if it is correct, good, and if not then he is redirected.