Newsflash: Need advice on what price to ask for from very interested buyer

So I own a lot of duplexes here in NYS and I normally don’t sell my properties since I get a a lot of equity and profit. Recently for one of my duplexes a guy reached out and stated the house used to belong in his family a while back(I verified this was legit per county records) and his parents lost the house at one point. This guy came forward and he’s very wealthy now and wants to offer whatever I wanted for it. I did lie and say a few people recently have been offering on it and they are offering way over market value but how do I just flat out present a much higher price to him? The house I have 40k remaining on mortgage and it’s worth roughly around 450k right now. I want to ask for way more since I know this guy can definitely pay it and due to the emotional connection he could justify it. I don’t know like I have a lot of real estate and done a lot of deals but I’ve never actually been on the selling side ever, I’ve always been the guy buying and I only had one property ever that was able to warrant ridiculous offers from interested parties but even in that time I just ended up developing the land rather than sell to developer.