Relieving and pay back option

Hi, I have been working with one of the largest Telecom operators for the past 2 years. Due to location constraints and growth restrictions I looked out for other opportunities and I succeeded in finding a good opportunity. As per the company policy I have to serve 2 months notice period or I can pay in lieu of the notice period which is company's discretion. I have to join the new company in 3 weeks time and I have informed my present employer that I am willing to pledge my salary for the rest of the notice period. Now my employer has denied me pledging of my salary and insists on serving the entire notice period. Can a company deny this when it's mentioned in the joining agreement and is their any legal standpoint to this. Please help. Thank you