I know you won't consider about his brought price. But you have to make sure it is a reasonable price. Otherwise, you can ask for $50M or as lower as possible.
You can try 90 of cause, but pls. think about 90(selling price) - 10 (Decoration) - 5 (Handling + others) = 75.
That's mean it will be nothing when he sold out the house if he has mortgage.
So, I think you can check his brought price and - 10%, then you will know the reasonable price. Also you can consider the seller situation for your final decision.