Mortgage Mod then take sub2.

Has anyone here been successfull with this strategy?
You have a house with 120k balance worth 90k. You get P&S agreement signed saying when they get their loan modified with the principal lowered to current maket value and the payment lowered you will take over sub2. (I had my loan moded and they didn’t lower the principal. Infact they added 5k to it and I’m upside down like 40k-50k) What is the best way to get the principal lowered to where the current market value is? Is there a professional that can do this process and get paid after their service so they or you don’t get screwd?