http://revach.net/article.php?id=1078

Section: Halacha   Category: Tshuvos
Buying An Esrog - The Arizal Would Put All His Money On The Table

The Zohar says that you should not make a bracha on any mitzva that you received for free, since it has on it a "Ruach Tumah". The Zohar says that Ashmadai the king of the Shaidim, told Shlomo HaMelech that in order to remove the tumah from a mitzva you must pay the full price that the seller requests. The Kaf HaChaim says that sometimes the Arizal would even place money on the table and tell the seller to take as much as he wants. In his introduction, the Sefer Charedim counts paying full payment as one of the 17 requirements of doing a mitzva in its complete form.