(1) If the performance was intended to produce a result the occurrence of which, according to the substance of the legal transaction, was regarded as uncertain, then, if the result does not materialise, the recipient is under a duty to surrender in the same way as if the claim to surrender had been pending at the time of receipt. The same applies if the performance has been rendered for a legal reason which according to the substance of the legal transaction was regarded as likely to lapse and the legal grounds cease to exist.
(2) The recipient is to pay interest only from the time onwards at which they learn that the result has not materialised or that the legal grounds have ceased to exist; they are not obliged to surrender emoluments to the extent that they no longer are enriched at this time.