PayPal - PHP API，自定义webform，付款失败，没有任何错误

Question

I've got a webform, sending data (and the customer) to PayPal. My JSON request:

{
"intent":"sale",
"payer":{
    "payment_method":"paypal"
},
"redirect_urls":{
    "return_url":"http://dev1.url.de/payment/execute?method=paypal&price=1&order_number=123465&success=true",
    "cancel_url":"http://dev1.url.de/payment/execute?method=paypal&price=1&order_number=123465&success=false"
},
"transactions":[
    {
        "amount":{
            "currency":"EUR",
            "total":"1"
        },
        "item_list":{
            "items":[
                {
                    "name":"123465",
                    "currency":"EUR",
                    "quantity":1,
                    "price":"1"
                }
            ]
        },
        "invoice_number":"123465"
    }
]
}

and a PayPal response:

{
"id":"PAY-0YV50781X7702245FKVDWC5A",
"create_time":"2015-05-04T12:09:24Z",
"update_time":"2015-05-04T12:09:24Z",
"state":"created",
"intent":"sale",
"payer":{
    "payment_method":"paypal",
    "payer_info":{
        "shipping_address":{

        }
    }
},
"transactions":[
    {
        "amount":{
            "total":"1.00",
            "currency":"EUR",
            "details":{
                "subtotal":"1.00"
            }
        },
        "invoice_number":"123465",
        "item_list":{
            "items":[
                {
                    "name":"123465",
                    "price":"1.00",
                    "currency":"EUR",
                    "quantity":"1"
                }
            ]
        },
        "related_resources":[

        ]
    }
],
"links":[
    {
        "href":"https://api.paypal.com/v1/payments/payment/PAY-0YV70781X7192245FKVDWC5A",
        "rel":"self",
        "method":"GET"
    },
    {
        "href":"https://www.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-38Y65870AE087743W",
        "rel":"approval_url",
        "method":"REDIRECT"
    },
    {
        "href":"https://api.paypal.com/v1/payments/payment/PAY-0YV50781X7170245FKVDWC5A/execute",
        "rel":"execute",
        "method":"POST"
    }
]
}

And going through the payment process on the PayPal website seems all to be fine. I can login, I can see all correct details and after I click on send payment, it redirects me to my form (return_url) and it says I paid.
The thing is .. it's not paid.
I can't see the money sent and it never arrives - not transaction is triggered. How can I figure out the problem? Or does anybody have a clue?