[For more current case annotated texts by this author, see Bernstein & Lookofsky, Understanding the CISG in Europe, 2d ed. (2003) and Lookofsky, Understanding the CISG in the USA, 2d ed. (2004).]
319. The Convention adopts a corollary to the restitutionary principle which is generally accepted in domestic systems of law: to avoid unjust enrichment, a party who is required to make restitution must also account for benefits received.
Thus, if the seller is bound to refund the price, he must also pay interest on it, from the date on which the price was paid.[l] The Convention does not set forth the rate of interest, thus leaving the calculation to domestic law. The interest calculation may vary in cases where the party claiming restitution is not the party in breach.
The benefits received principle applies to the buyer as well: if the buyer must make restitution of the goods or in whole or part, she must account to the seller for all benefits which she has derived from the goods or part of them. The buyer must also account to the seller for all benefits derived in cases where it is impossible for her to make restitution of all or part of the goods substantially in the condition received, but where she nevertheless has declared the contract avoided or required the seller to deliver substitute goods.