Even if the money had absolutely nothing to do with the deal itself and wasn't even US money, the optics of it were still terrible due to the timing.
As the common complaint was that it
wasn't to do with the Deal, but with the release of US people from Iran, this isn't actually as bad, and in line with the slightly reduced set of sanctions being a "you play ball, we play ball" thing.
If people want to complain that it was part of the Nuclear Deal, that's a distinct improvement in worldview, but they still ought to know more (it was a much lower amount than had been claimed as rightfully theirs by Iran, by a magnitude or so).
Either way, being a disconnected thing, would "I'm sorry, we can't pay you this money that we owe you
right now that has nothing to do with the very restrictive Deal that you've just signed up to, because it would look like we'd paid you for this. In order not to be seen to reward you for taking this deal, we're going to have to further punish you for taking this deal by delaying payment until a time it doesn't look like a reward" (at which time, if choosing a perfect nadir of future relations, far away from any past or approaching good relations of note that might have 'bad optics', now the optics show that the dog is being given its juicy bone for
bad behaviour). You just can't get it right
ever, in this relatively honourable endeavour, even with perfect hindsight!
I get a bit annoyed by the Trump Gets It Wrong Again autotweets/etc that use this appoach (there's a a huge bunch of Trump Can't Stop Winning For Winning ones too, though, so
balance...), although he doesn't help things by declaring the creation of Chinese jobs in a Chinese firm in China (whose products have incidentally been barred by the US military as a data-security threat, and more) a 'win' for MAGA. Must be
really desperate to obscure something else in the news to try to promote that talking point, based on his proven track record. I
might go and find out what.