The field of restitution, broadly considered, involves all those situations in which a person who holds property (or has consumed it) must deliver it (or its value) to the claimant in order to prevent the unjust enrichment of the holder. In this sense the ancient common law writs for the recovery of chattels or their value (detinue, replevin, and trover) and land (ejectment) are perceived to be restitutionary in character. A more modem development in the law courts, the allowance of quasi-contractual relief upon the common counts in general assumpsit, rests upon the same basis. In a leading English case, Lord Mansfield states the true basis of the latter obligation.
If the defendant be under an obligation, from the ties of natural justice to refund, the law implies a debt, and gives this action founded in the equity of the plaintiff's case, as it were upon a contract ('quasi ex contractu') as the Roman law expresses it … This kind of equitable action, to recover back money, is very beneficial, and therefore much encouraged. It lies only for money which, ex aequo et bono, the defendant ought to refund.
Contract sounded in promise but quasi-contract had its roots in the notion of unjust enrichment.
Banking and Finance Law | Law
Henry P. Monaghan,
Constructive Trust and Equitable Lien: Status of the Conscious and the Innocent Wrongdoer in Equity,
U. Det. L. J.
Available at: https://scholarship.law.columbia.edu/faculty_scholarship/4075