Reliving issue

Hi,

I am working in IT service company, Recently I got an offer with good Hike,now want to join new company.  In my current companies offer later its written that

"You will be liberty to resign from the service of the company subject minimum on month notice in writing to the company or one moth salary in lieu thereof,

Now I have given them(current company) 15 days notice and ask them take 15 days salary but they are not accepting it. pls suggest its very important for my carrier.