Worth a Hill of Beans (a totally gen fairy tale)

Tonight in email I mentioned to my partner-in-crime the traditional story in which a cow is traded for magic beans. The man who traded the beans knew they were magic & potentially worth a fortune, so why was he willing to trade them for a cow? Unless he thought it wasn't an ordinary cow.

( around 400 words, ok, sentimental, if you need warning for that. :^) )