First we have to see who's talking to whom saying "Sorry, not now, I am too busy".
A boss towards an employee would say "Sorry, not now", but probably wouldn't add "I am too busy", not having to explain why this isn't the right time to interrupt.
An employee towards the boss would probably put it at all differently.
But an employee would say that to another employee or to a family member or a friend. This implies a more informal or colloquial expression, e.g.
Tut mir leid, im Moment geht es nicht, ich habe zu tun
To show your obligation you can add "gerade" or "im Moment" to express that the occupation is temporary and you will soon be ready to divert your attention to the person you're talking to (already included in the example above, don't use it twice in a sentence), and you can add "viel" to express that your workload really has to be your primary concern right now.