How to Set Up a Reference Transaction Using Express Checkout.curl

CreateBillingAgreement

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=insert_merchant_user_name_here
&PWD=insert_merchant_password_here
&SIGNATURE=insert_merchant_signature_value_here
&METHOD=CreateBillingAgreement
&VERSION=86
&TOKEN=insert_token_value_here 

Response
--------
BILLINGAGREEMENTID=B%2d7FB31251F28061234    
&ACK=Success
...

DoReferenceTransaction

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=insert_merchant_user_name_here
&PWD=insert_merchant_password_here
&SIGNATURE=insert_merchant_signature_value_here
&METHOD=DoReferenceTransaction
&VERSION=86
&AMT=50    #The amount the buyer will pay in a payment period
&CURRENCYCODE=USD    #The currency, e.g. US dollars
&PAYMENTACTION=SALE   #The type of payment
&REFERENCEID=B-7FB31251F28061234   #Billing agreement ID received in the DoExpressCheckoutPayment call

Response
--------
BILLINGAGREEMENTID=B%2d7FB31251F28061234
&ACK=Success
&TRANSACTIONID=98A13946GS4491234
&TRANSACTIONTYPE=merchtpmt
&PAYMENTTYPE=instant
&AMT=50%2e00
&PAYMENTSTATUS=Completed
...

SetExpressCheckout

Request 
-------
Endpoint URL: https://api-3t.sandbox.paypal.com/nvp
HTTP method: POST
POST data:
USER=insert_merchant_user_name_here
&PWD=insert_merchant_password_here
&SIGNATURE=insert_merchant_signature_value_here
&METHOD=SetExpressCheckout
&VERSION=86
&PAYMENTREQUEST_0_PAYMENTACTION=AUTHORIZATION    #Payment authorization  
&PAYMENTREQUEST_0_AMT=0    #The amount authorized
&PAYMENTREQUEST_0_CURRENCYCODE=USD    #The currency, e.g. US dollars
&L_BILLINGTYPE0=MerchantInitiatedBilling    #The type of billing agreement
&L_BILLINGAGREEMENTDESCRIPTION0=ClubUsage    #The description of the billing agreement
&cancelUrl=http://www.yourdomain.com/cancel.html    #For use if the consumer decides not to proceed with payment 
&returnUrl=http://www.yourdomain.com/success.html   #For use if the consumer proceeds with payment 

Response
--------
TOKEN=EC%2d2B984685J43051234
&ACK=Success
...