@pepsi_man @Bad_Banner @Wormwood The 10% thing can be found here (Genesis 28:20-22):
Then Jacob made a vow, saying, “If God will be with me and will keep me in this way that I go, and will give me bread to eat and clothing to wear, 21 so that I come again to my father's house in peace, then the Lord shall be my God, 22 and this stone, which I have set up for a pillar, shall be God's house. And of all that you give me I will give a full tenth to you.”
Gotcha.
So it's just like the circumcision thing then. There's no specific command to do it, but American evangelicals have (once again) misinterpreted scripture.
Really, with regards to the 10% bit, I'd refer folks to a story called "The Richest Man in Babylon."