earnest money

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Money paid as partial payment of a purchase price in order to establish a binding contract of sale.

from The Century Dictionary.

• noun Money paid as earnest to bind a bargain or ratify and confirm a sale. Also called hand-money.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Money paid as a deposit to show intent to buy or to reserve an item to be purchased; especially, money accompanying an offer to buy real estate.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun money given by a buyer to a seller to bind a contract