Notice period payment

Dear All, I had worked in a MNC for six years where I was having the agreement of two month's notice period. After six years, in my last increment letter management has changed the notice period from two months to three months for which I have shown my disagreement and have not signed the increment letter. However I had sent a mail attaching the signed increment letter and mentioning the note that new changes in notice period are not accepted to me. My earlier company never replied my mail. Now when I resigned and left the company after serving two month's notice period, they deducted my one months salary saying that, as per present policy notice period is of three months. As per HR, since I accepted the revised salary all the terms in the increment letter get automatically accepted by me although I had sent mail for not accepting the same. Can anybody guide me on the basis of above situation whether my ealier company have any right to deduct my one months salary for notice period? Also what should i do to get my money which is deduted by my earlier company? Thanks