Well, times change, so well - laws change and society can renegotiate laws if they oppposse the common good. Nothing written in stone.
The Contracts Clause: The Contracts Clause of the U.S. Constitution (Article I, Section 10, Clause 1)
> No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title ...
You could still take it back with eminent domain probably, but would have to compensate I think at current value, not at bad negotiated value from some time ago.