Article 84 CISG

(1) 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.

(2) The buyer must account to the seller for all benefits which he has derived from the goods or part of them:

if he must make restitution of the goods or part of them; or
if it is impossible for him to make restitution of all or part of the goods or to make restitution of all or part of the goods substantially in the condition in which he received them, but he has nevertheless declared the contract avoided or required the seller to deliver substitute goods.