Bauer’s Lexicon defines ἀρραβών as “payment of part of a purchase price in advance; first installment, deposit, down payment, pledge”. In time, the meaning has shifted to the kind of pledge associated with marriage: a betrothal, an engagement.
(Greeks, please do not cite Ancient words with Modern inflections. It’s just confusing to those not as blessed as you to speak Modern Greek.)
The word actually entered Greek in the classical era; it is used by the Attic orators like Isaeus and Antiphon; so it would likeliest have come in from Phoenecian, not Hebrew. Liddell-Scott does indeed cite the Hebrew as ’ērābōn.
And that word is 6162. עֲרָבוֹן (erabon) — a pledge .