save node after paypal payment success
Get the response from Pay-pal if the response code is 200 save the node otherwise don't save it.

for example :

if ($responseCode == 200){
  //node_save($node);
}
else {
  print "payment failed";
}


https://www.drupal.org/node/1207878
check this for more info only for condition to check the payment status.