Callback to Drupal site after curl call to 3rd party site doesn't resume authenticated session if more than 120 seconds elapsed since authentication
A callback from a credit card processing site to your Drupal server should never have the same session as the paying customer. If it does, then you have a problem.
But I'm not sure that's what you're saying - you are vague when you talk about session ID. If you mean that the paying customer's session ID shouldn't change when your server sends the payment information to the credit card processing site then receives the response, then you are correct. But why even mention credit card processing and posting back in this case, as it's not very relevant? If the paying customer's session ID changes, then you need to investigate how the session ID is invalidated and what your session timeout is. Perhaps your callback is invalidating a cache tag that forces a new session to be generated for that user.