Русско-англо-русский юридический словарь

In law, unjust enrichment is where one person is unjustly or by chance enriched at the expense of another, and an obligation to make restitution arises, regardless of liability for wrongdoing. A common example is when a party contracts to provide a service, but the contract is terminated prematurely due to a breach, and the contractor unjustly receives no compensation for partial services rendered.

The concept of unjust enrichment is based upon the Roman legal maxim "no one should be benefited at another's expense" (nemo locupletari potest aliena iactura or nemo locupletari debet cum alien iactura). (Wikipedia)

“Non-contractual obligations” can include tortious claims, such as negligence, misrepresentation and pre-contractual deceit or claims in respect of restitution or unjust enrichment. (lexology.com)

The basic principle governing reparations is restitutio in integrum or “full restitution.” ( website of the International Center for Transitional Justice)

Mutual restitution has the force of a general rule: it will apply unless special rules provide for other legal consequences. In its simplest form mutual restitution means that each partner must return to the other everything received under the transaction, for example, the purchaser will return the thing bought and the seller will return the money paid. (Soviet Civil Law)