But if the animal was not as advertizes that is the breeders wrong and not a miscare from the buyer. Though the good thing to do was to talk to the breeder, it does not change that the snake could not have completely changed size from somwthing done on arrival.
I dont know how breeder in the herp community do it, but thats a one way sur way to get bad rep.
Im sad for the snake but as said above, take the money, and never go there again. Bext time theres a problem say it to the breeder. If then they dont take responsability, you might have something to go against them but right now you only have what you say. A necropsy is also an option (thats the first thought I had)