Bribe, ransom, debt repayment? I don't really care. What I do care about is what is State actually trying to "buy" with this 400 million dollars? If it's just hostages then it just reinforces the idea that the United States is playing checkers while more sophisticated nations are playing chess, when it comes to international affairs.
That being said, Persia/Iran does have some legitimate grievances with the U.S. (All of that CIA-British Petroleum nonsense back in 1953 and then propping up the Shah over a democratically elected government and everything else that followed afterward as blowback -- the revolution, the rise of Islamic fundamentalism, etc.). If this is part of a strategic shift away from Saudi Arabia then I don't know if that's a good or bad thing, because I'm not an intelligence analyst and privy to the behind-the-scenes stuff, but off-hand I can see good and bad reasons for trying to soften some of the ill-will between Iran and the U.S, particularly with the amount of instability in various Sunni regimes and the subsequent rise of Sunni ISIS/Al-queda terrorist networks around the world, the simple fact is that we're probably going to need a strong Shia regional power (and that ain't Iraq) as an ally to act as counterweight and a stabilizing force in the region.
Dangerous waters ahead.