Postfix to deliver local the relay.
 

I have one domain example.com
Some mail coming to my postfix server will need to be delivered local and the rest to an internal mail server.

For example...
bob@example.com is in the alias file to go to root
bob: root

sam@example.com is not in the alias file and needs to be relayed to the internal server.

The internal mail server is mail.example.com where there is no bob@example.com but there is a sam@example.com.

Is it possible to configure postfix to do this?

There are probably several ways you can do this. One is to use fallback_transport.
You need to add the remote recipients to local_recipient_maps as shown here. Even though it's not convenient to list all recipients explicitly (and there are ways around it), it's best practice not to accept mail unless you know it will be delivered ultimately.