Erreur HTTP 400 Bad Request lors de la Publication des données JSON sur HttpURLConnection

J'ai ce code ci-dessous qui essaie de publier des données JSON après avoir réussi à s'authentifier via Oauth2.0, le code ressemble à ceci:

try {            
    URL url = new URL ("https://test.api.neteller.com/v1/oauth2/token?grant_type=client_credentials");
    String test = "QUFBQlNhVlFoUjhlTVhPQjowLm9aV3g2Rkhwd1dIQUZnXzRaMDZwOUoyRWtzaXVHMHEzd2xrVFF4MkRVM28uN0FzUDE4cmZXU0dxVDdqWE1NUGhzSFY4cHU0";
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    connection.setRequestMethod("POST");
    connection.setDoOutput(true);
    connection.setRequestProperty  ("Authorization", "Basic " + test);
    String accessToken = "";
    [... get the access token ...]

    String urlnew = "https://test.api.neteller.com/v1/transferIn";
    URL obj = new URL(urlnew);
    HttpsURLConnection con = (HttpsURLConnection) obj.openConnection();

    //add request header
    con.setRequestMethod("POST");
    con.setDoOutput(true);          
    con.setRequestProperty  ("Authorization", "Bearer " + accessToken);
    con.setRequestProperty("Content-Type", "application/json");

    //Send post request            
    JSONObject obj2 = new JSONObject();
    obj2.put("paymentMethod", "");
    obj2.put("type", "neteller");
    obj2.put("value", "[email protected]");
    obj2.put("transaction", "");
    obj2.put("merchantRefId", "20140203122501");
    obj2.put("amount",  (5000));
    obj2.put("currency", "USD");
    obj2.put("verificationCode", "411392");
    System.out.print(obj2);             

    int responseCode = con.getResponseCode();
    System.out.println("Response Code : " + responseCode);

    [... parse response ...]
} catch(Exception e) {
    e.printStackTrace();
}

J'obtiens cette erreur quand je lance le code ci-dessus:

java.io.IOException: Server returned HTTP response code: 400 for URL: https://test.api.neteller.com/v1/transferIn
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at sun.net.www.protocol.http.HttpURLConnection$6.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.net.www.protocol.http.HttpURLConnection.getChainedException(Unknown Source)
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
    at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(Unknown Source)
    at example.demo.main(demo.java:71)
Caused by: java.io.IOException: Server returned HTTP response code: 400 for URL: https://test.api.neteller.com/v1/transferIn
    at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
    at java.net.HttpURLConnection.getResponseCode(Unknown Source)
    at sun.net.www.protocol.https.HttpsURLConnectionImpl.getResponseCode(Unknown Source)
    at example.demo.main(demo.java:65)

De l'état d'erreur 400 je sais que le post de données JSON n'est pas valide. Je soupçonne peut-être dans la structure, car, selon la documentation, il devrait ressembler à ceci:

{
    "paymentMethod": {
        "type": "neteller",
        "value": "[email protected]"
    },
    "transaction": {
        "merchantRefId": "20140203122501",
        "amount": 5000,
        "currency": "USD"
    },
    "verificationCode": "234124"
}

Alors, comment puis-je modifier le paramètre JSON afin qu'il puisse suivre la structure de la documentation?