is it possible to take over a "subject to" loan?

[Josh Fincher]

Someone is selling their house at about 20k to 30k less than other comparable homes in the neighborhood but they have a usda loan and are seeking payoff. He is trying to sell so he can buy his wifes childhood home. Is there a way to take over the loan with "sub to" terms. If so how and is it legal? This is my first property so any info is greatly appreciated.