The State cannot be unjustly enriched

A general equitable principle that a person should not profit at another's expense and therefore should make restitution for the reasonable value of any property, services, or other benefits that have been unfairly received and retained

Fibrosa v. Fairbairn (All ER p.135 H) “[A]ny civilised system of law is bound to provide remedies for cases of what has been called unjust enrichment or unjust benefit, that is, to prevent a man from retaining the money of, or some benefit derived from, another which it is against conscience that he should keep. Such remedies in English law are generically different from remedies in contract or in tort, and are now recognised to fall within a third category of the common law which has been called quasicontract or restitution.” 

In Sahakari Khand Udyog Mandal Ltd. v. CCE & Customs, reported at (2005) 3 SCC 738, Supreme Court of India (at page 748) elaborated upon the aspect of unjust enrichment: 

“31. Stated simply, “unjust enrichment” means retention of a benefit by a person that is unjust or inequitable. “Unjust enrichment” occurs when a person retains money or benefits which in justice, equity and good conscience, belong to someone else. 

32. The doctrine of “unjust enrichment”, therefore, is that no person can be allowed to enrich inequitably at the expense of another. A right of recovery under the doctrine of “unjust enrichment” arises where retention of a benefit is considered contrary to justice or against equity…”  

Godfrey Phillips India Ltd. & Anr. v. State of U.P & Ors. reported at (2005) 2 SCC 515.

In this case, the constitutional validity of the Uttar Pradesh Tax on Luxuries Act, 1995 was  challenged inter alia on the ground of legislative competence of the State Legislatures. The Court allowed the petition and held that the State Legislatures were not competent to impose luxury tax on tobacco and tobacco products and the Acts were declared ultra vires and unconstitutional. In the intervening period, however, tax was collected by the appellants from consumers and also paid to the State Governments. In certain cases, interim relief was obtained by the appellants from this Court against recovery of tax and as alleged by the State Governments, the appellants continued to charge tax from consumers/customers. The Court held:  

It was stated on behalf of the State Governments that after obtaining interim orders from this Court against recovery of luxury tax, the appellants continued to charge such tax from consumers/customers. It is alleged that they did not pay such tax to respective State Governments. It was, therefore, submitted that if the appellants are allowed to retain the amounts collected by them towards luxury tax from consumers, it would amount to ‘unjust enrichment’ by them. In our opinion, the submission is well founded and deserves to be upheld. If the appellants have collected any amount towards luxury tax from consumers/customers after obtaining interim orders from this Court, they will pay the said amounts to the respective State Governments.” 

North Dakota Supreme Court, in Ritter, Laber and Assocs., Inc. v. Koch Oil, Inc., 2004 ND 117,  26, 680 N.W.2d 634 and 2008 ND 189 Lord & Stevens, Inc., dba Express Press, Plaintiff and Appellee and Michael and Jenny Stevens, Involuntary Plaintiffs v. 3D Printing, Inc., Defendant and Appellant and MBA Investments Three, LLC, Defendant, has laid down the following elements for unjust enrichment:

1. An enrichment
2. An impoverishment
3. A connection between enrichment and the impoverishment
4. Absence of a justification for the enrichment and impoverishment

Contract by Conduct.

Baltimore & Ohio Railroad Co. v. United States, 261 U.S. 592 (1923

“an agreement … founded upon a meeting of minds which, although not embodied in an express contract, is inferred, as a fact, from conduct of the parties showing, in the light of the surrounding circumstances, their tacit understanding.”