1. Yes, it is compulsory to serve the 90 days notice period, if it is in the employment terms
2. Management has the discretion to waive the notice period service or salary recovery in lieu
3. There is no automatic right for an employee to trade salary payment in lieu of notice period service
The above is the legal position. IT companies are "tightening" their dealings with employees these days; IT is no more a "seller's market".
Finally, if you desire a long career, be an ethical professional; you will earn respect and this will more than repay short term gains.
Best of luck in career.
