pot-de-vin

from Bouvier's Law Dictionary, Revised 6th Ed (1856)
POT-DE-VIN, French law. A sum of money frequently paid, at the moment of 
entering into a contract, beyond the price agreed upon. 
     2. It differs from arrha, (q.v.) in this, that it is no part of the 
price of the thing sold, and, that the person who has received it, cannot by 
returning double the amount, or the other party by losing what he has paid, 
rescind the contract. 18 Toull. n. 52. 
    

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