Trending @Accenture; Buyout option

I am holding an offer from another company and they want me to join within 45 days and are ready to help with the buyout. I am currently in a project as a QA and its not even my primary skill, i have just been trained and added to it. Considering my previous 4 months being in bench, i am not going to get promotion or hike as well. How do i approach HR for buyout option and what would be the best arguements to make. I would appreciate some guidance as this is new to me.