Notice period buyout

Dear Sir/Madam, I have Resigned from my current organisation. I am ready to pay by out amount but current employer is not ready for my notice period by out. in the agreement that I have signed had following clause. “ On confirmation of your service, either party may terminate this contract by giving to the other three months notice of termination or three months salary in lieu thereof. If adequate notice as aforesaid is not given by an employee while resigning from the service of the company, appropriate direction will be made to cover the notice period from out of the amounts Payable to him by the company in the final settlement of accounts. However the decision on adjusting the notice period will be based on the sole discretion of the management. only basic salary will be taken into account for payment of salary in lieu of notice period.” So is it possible for me to buy out the notice period? I am already resigned yesterday and have informed about the buyout to the management but they are saying it’s mandatory to serve 90 days as its mention in offer letter stating “decision on adjusting notice period will be based on the sole discretion of the management”. Please help me on the below.