This seems a little 'grass is greener' to me. Not sure if this is glib, but:

You can build the business again from scratch (ok, maybe you have a non compete for a while...) But you can't magic the $ you just got, out of thin air.

So he's been paid for the hard work he has done, and now if he so chooses he can do the same again, or different.

So seems like Boris is standing on the greener grass right now, to me at least. I think the post-sale reflection is naturally something that will make people question their decisions... It may be possible for Boris to Stoically rearrange his view here. There's definitely a bright shiny alternative to regret/second-guessing.