I need to know how to fill out " subject to " agreement

[Danny Pollard]

When filling  a contract " subject to " is the purchase price the existing  amount of the loan , or is the existing loan apart of the purchase price . Does the loan amount go on the purchase price or what ?Can someone show  me an example of how these transactions are put together .   

So if the loan amount is if the seller got a loan for 125k for the home gas paid 15k so now they owe 115k how do i go about the negotiation and what would you fill out on the subject to form