term:

Unjust Enrichment

unjust enrichment n

1 : the retaining of a benefit (as money) conferred by another when principles of equity and justice call for restitution to the other party
;also
: the retaining of property acquired esp. by fraud from another in circumstances that demand the judicial imposition of a constructive trust on behalf of those who in equity ought to receive it see also quasi contract 1 at contract
2 : a doctrine that requires an equitable remedy on behalf of one who has been injured by the unjust enrichment of another