PayPal reference transaction- status code 400 in response - php

Question

I try to use PayPal Reference Transactin. I have already made a billing agreemet, got the Billing Agreement ID set up before any transaction is done.
I make a simple transaction request, but got:
PPConnectionException in PPHttpConnection.php line 108:
Got Http response code 400 when accessing
https://api-3t.sandbox.paypal.com/2.0.
My code (writte in Laravel 5.3 with merchant-sdk-php package):
$user=Auth::user();
$currencyCode = 'GBP';
$price=config('constants.offer_submit_price');
$reference_id=$user->userExt()->where('field_name', '=', 'billing_agreement')->firstOrFail()->field_value;
$amount = new BasicAmountType($currencyCode, $price);
$paymentDetails = new PaymentDetailsType();
$paymentDetails->OrderTotal = $amount;
$RTRequestDetails = new DoReferenceTransactionRequestDetailsType();
$RTRequestDetails->PaymentDetails = $paymentDetails;
$RTRequestDetails->ReferenceID = $reference_id;
$RTRequestDetails->PaymentAction ='sale';
$RTRequest = new DoReferenceTransactionRequestType();
$RTRequest->DoReferenceTransactionRequestDetails = $RTRequestDetails;
$RTReq = new DoReferenceTransactionReq();
$RTReq->DoReferenceTransactionRequest = $RTRequest;
$paypalService = new PayPalAPIInterfaceServiceService($this->config);
try {
$setRTResponse = $paypalService->DoReferenceTransaction($RTReq);
} catch (Exception $ex) {
dd($ex);
}
Thats the SAOP envelopr produced by script:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns="urn:ebay:api:PayPalAPI" xmlns:ebl="urn:ebay:apis:eBLBaseComponents" xmlns:cc="urn:ebay:apis:CoreComponentTypes" xmlns:ed="urn:ebay:apis:EnhancedDataTypes" >
<soapenv:Header>
<ns:RequesterCredentials>
<ebl:Credentials>
<ebl:Username>username
</ebl:Username>
<ebl:Password>password
</ebl:Password>
<ebl:Signature>signature
</ebl:Signature>
</ebl:Credentials>
</ns:RequesterCredentials>
</soapenv:Header>
<soapenv:Body>
<ns:DoReferenceTransactionReq>
<ns:DoReferenceTransactionRequest>
<ebl:DoReferenceTransactionRequestDetails>
<ebl:ReferenceID>B-reference_id
</ebl:ReferenceID>
<ebl:PaymentAction>sale
</ebl:PaymentAction>
<ebl:PaymentDetails>
<ebl:OrderTotal currencyID="GBP">10
</ebl:OrderTotal>
<ebl:ButtonSource>PayPal_SDK
</ebl:ButtonSource>
</ebl:PaymentDetails>
</ebl:DoReferenceTransactionRequestDetails>
<ebl:Version>106.0
</ebl:Version>
</ns:DoReferenceTransactionRequest>
</ns:DoReferenceTransactionReq>
</soapenv:Body>
</soapenv:Envelope>'

Answer 1

With help of my friend I have figured out what was wrong.
$RTRequestDetails->PaymentAction ='Sale';
instead of
$RTRequestDetails->PaymentAction ='sale';
PayPal SOAP API need to have everything in right case sensitive.
Regret, they don't send any info what's wrong.