Looking to purchase a property "subject to" and have questions

[Sarah Robinson]

So I've been looking into "subject to" deals because I actually have a friend who is in a position where he has to move across the country and can't seem to sell his house. I have two conventional rentals but I have never really looked into acquiring a property using a "subject to" clause. I have a few questions for those of you who have been involved in this before:

How does the seller make money on the deal? Is it common for the buyer of the property to pay the "asking price" for the house above the amount still owed on the mortgage? If that is the case (lets say a $40,000 difference), how would one "normally" go about compensating the seller (lump sum, payments, etc). I know that the original financing remains in place and you as the buyer assume the loan payments, but I guess I'm asking how the second part of that transaction looks (the seller making $40,000 in my example).

Also, with interest rates rising, how would you rate the likelihood of the original mortgage company "calling" the loan? I think this is my primary concern when looking at this kind of deal. I know it's always a risk, but I guess I'm looking to get feedback from people with more experience than myself at this. Has anyone had this happen, and if so what did you do? 

Thank you in advance for your help on this!