If the guy wasn’t aware of such a pending contract when he negotiated the deal, well there’s something wrong there too.
I won't argue that.
But we really don't have any facts here. And it does happen that things suddenly take off beyond anyone's realistic expectations.
Personally, I see nothing unethical in requesting a renegotiation of the price if the lengthy delay was on the side of the buyer. And that would depend upon what kind of terms were set down at the time of the pending agreement. Lots of variables we may not know about.