Earnest money

money paid as security

An earnest payment (sometimes called earnest money or simply Earnest, or alternatively a Good-faith deposit) is a deposit towards the purchase of real estate made by a buyer to demonstrate that he/she is serious (earnest) about wanting to complete the purchase. When a buyer makes an offer to buy residential real estate, he/she generally signs a contract and pays a sum acceptable to the seller by way of earnest money. The amount varies enormously, depending upon local custom and the state of the local market at the time of contract negotiations.
See more at Wikipedia.org...

Noun
1. money given by a buyer to a seller to bind a contract
(hypernym) earnest

The money given to the seller by the buyer to demonstrate the buyer's good faith. If the deal falls through, the deposit is usually forfeited.