The Collaborative International Dictionary of English v.0.48:

Arrha \Ar"rha\, n.; pl. Arrh[ae]. [L. Cf. Earnest.] (Law)
   Money or other valuable thing given to evidence a contract; a
   pledge or earnest.
   [Webster 1913 Suppl.]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ARRHAE, contracts, in the civil law. Money or other valuable things given by
the buyer to the seller, for the purpose of evidencing the contract earnest.
     2. There are two kinds of arrhae; one kind given when a contract has
only been proposed; the other when a sale has actually taken place. Those
which are given when a bargain has been merely proposed, before it has been
concluded, form the matter of the contract, by which he who gives the arrhae
consents and agrees to lose them, and to transfer the title to them in the
opposite party, in case he should refuse to complete the proposed bargain;
and the receiver of arrhae is obliged on his part to return double the
amount to the giver of them in case be should fail to complete his part of
the contract. Poth. Contr. de Vente, n. 498. After the contract of sale has
been completed, the purchaser usually gives arrbae as evidence that the
contract has been perfected. Arrbae are therefore defined quod ante pretium
datur, et fidem fecit contractus, facti totiusque pecuniae solvendae. Id. n.
506; Code, 4, 45, 2.