Buy out notice period from company

Hi, I'm IT professional from Pune, Maharashtra, I'm in situation where my next company wanted me to join in 1 month but my current employer will not allowed to do so. Current employer has 2 months notice which they can extend to 3 months as well. One positive thing is they have mentioned in offer letter that we can buy our notice period bye giving them last drawn salary. But here is one problem in last months I got incremented salary from April, i.e 4 months incremented salary in last month. So in this scenario what I have to pay really as last drawn salary, salary for one moth after increment or that 4 month salary.