Law exam answers (or bar exam answers for that matter) are hardly ever geared toward real life instances. I don't think I've ever heard of someone shooting at their wife, with the intent to kill her, missing, having the bullet go out the window and killing a guy who jumped off the roof of the building (committing suicide). And then the guy being found guilty for 1st degree murder under transferred intent.
Basic thing to take out of this problem is (1) offer: "do you want to buy my goat for $10" (2) acceptance: Could have just said "yes" and would have been enough. Here, he slapped the $10 down (red herring that he "slapped" it down) (3) $10 was consideration for the goat: B gave consideration and A gave something up for it (detriment).
Maybe I'm sounding like a law student now. All I do is deal cards on a boat. LOL