Issue with Project ID association to customer in the Admin

I am creating customers with the new Xentral API In our Environment and the customer is successfully created. However, I am failing to assign a the project ID to customer and  in the Admin Interface project id is empty.

{
    "customer": {
        "contact": {
            "email": "test@web.de",
            "marketingMails": false,
            "phone": "",
            "trackingMails": false
        },
        "general": {
            "address": {
                "city": "Hamburg",
                "country": "DE",
                "note": "",
                "street": "Roman-Platz 10",
                "zip": "22437"
            },
            "name": "Esma Nur Efe"
        },
        "project": {
            "id": "8003"
        },
        "type": "person"
    }
}

and these are the projects in our sandbox

{
    "data":  
        {
            "id": "2",
            "name": "CH",
            "keyName": "3005",
            "description": "",
            "currency": "EUR",
            "normalTaxRate": 19,
            "reducedTaxRate": 7,
            "storageProcess": "none",
            "pickingProcess": "none"
        },
        {
            "id": "4",
            "name": "shop",
            "keyName": "8003",
            "description": "",
            "currency": "EUR",
            "normalTaxRate": 19,
            "reducedTaxRate": 7,
            "storageProcess": "none",
            "pickingProcess": "shipmentNote"
        }
    ],
    "extra": {
        "totalCount": 2,
        "page": {
            "number": 1,
            "size": 10
        }
    }
}

Any help appreciated