Business etiquette in Sri Lanka

I am happy to share my exoetience because I was once a local working in Sri Lanka and I am now back as expat 30 years later. But been coming often to Sri Lanka meeting local and foreign professionals.

1.  For instance, are there office manners?
Greet everyone Good Morning when you arrive and do it with a smile. 
2. How do you greet your co-workers?
As above friendly as equals
3.Do you greet your management differently?
Yes more formally and respectfully in front of the staff and can be more informal privately.
4. Is there a dress code?
Differs office to office. But to dress more conservatively but slightly less than your immeduate manager.
5. Particular rules to observe? Never question your boss or didagree in front of staff. Dont be rude to colleagues.
6. Maybe a professional body language?
Yes do not put your feet up on the table and blow your nose quietly unlike in the west or do it privately.
7. On another level, what is key for a successful professional meeting?
Most meetings are for a show and achieve very little. But get to know all well before meetings. Be clear in your exoectations on actions and be clear you expect results and you will follow up.
8. Are there any steps to follow? How do you a start a negotiation?
Praise the other party for their interest and expertise.
9. In other words, what are the most important things to know for a successful professional integration in Sri Lanka?
Dont mistake the respectful nature of Sri Lankans towards foreigners as a feeling of inferiority. Respond to that by respecting them as your equal. Always demonstrate by your actions and words you feel they are equal and as good as you.   

Remember Sri Lankans generally do not have the individualism of the West. They lack the sense of responsibility and accountability and expect the boss to take that upon himself or herself for them. So explain what you expect of them.

Be warm, smile be friendly and you will be welcomed and integrated.

Dear Raja!

Thank you very much for this valuable answer.