I strongly disagree....the government with which we made that deal is gone forever...with it our obligation IMO.
That is not typically how we treat the matter, though, is it? For example, say we shipped the UK some new F-35 fighters in early 2016, in exchange for a promise to pay us in 2017. Only, in between, they got a new government:
http://www.bbc.com/news/uk-politics-36788782
And, arguably, we did, too.
Would we regard their payment obligation as void because the deal was made with prior governments? Of course not. Even when there's a "new government" in a more radical sense, we regard such obligations as enduring. For example, lots of African and Central American nations have debts run up by various deposed regimes, but they're still expected to pay on them.
If we took the position that a change of regime was all that was needed to throw past contractual obligations into question, we'd end up worse off far more often than we'd end up better off, since all sorts of countries that owe us could use that loophole as a way to void a debt.
There was no pressing need either morally or financially to restore that money to the current regime in power
There was a practical reason to do so. Iran was on the path to becoming a nuclear power, and short of starting a devastating and illegal war of aggression against them, against the protests of most of the world, there wasn't much we could do about it. The negotiated deal opened them up to nuclear inspections and put other leading nations in a position where they had an interest in enforcement.
We saw where the other path leads, during the Reagan/Bush era, with regard to North Korea. They pretty much ignored North Korea's move towards nuclear potency, imagining that tough words and isolation would magically keep North Korea from getting nukes, and that, of course, failed. By 1993 North Korea had at least a couple nuclear warheads, and was poised to start making dozens per year, thanks to the near completion of a couple large reactors at YongByon. Fortunately, Clinton's deal took them off that course, but some of the key damage had already been done by the Reagan/Bush team's criminal incompetence -- North Korea was a member of the nuclear club, and that genie isn't going back in the bottle.