I found a buyer who wants me to buy a house for under 50,000.
So the deal would be that "you" buy this house using your GrandFather's money, and then hope that the Buyer will buy it from you.
I hope you have the same taste buds as the Buyer . . . . you don't want him getting cold feet or changing his mind because I don't think your GrandFather will be too interested in a 2nd experience if you get stuck with it.
Why not just sell your interests in the Contract to Buy? That way, you never take possession or incur any Closing Costs . . . . let the real Buyer pay those if he remains interested.
I'd be wary that your "Buyer" may also be a flipper, whose looking for such a good deal that he can quickly sell it to another ready and willing Buyer . . . . but just like in Musical Chairs; someone will be left without a seat.
He may just use you as a Bird Dog.