Русско-англо-русский юридический словарь

Another civil law classification relates to the advantage each party expects to receive from the agreement. A contract, such as a sale of goods, in which both parties expect to receive an advantage in exchange for the service or item they provide, is considered onerous. A contract where one party provides an advantage without receiving anything in return, such as a donation, is a gratuitous contract. (Civil Law Contracts by Christopher Meldrum)

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)

Though Charles has waived his usual author's fee, his charitable foundation will receive undisclosed royalties. (hellomagazine.com)

A letter of credit is a document from a bank guaranteeing that a seller will receive payment in full as long as certain delivery conditions have been met. (Wikipedia)