Granted but it's as academic as the idea that Mando is going to literally kill him over this. Greef is confident he can afford to rob Mando of some of his money because Mando needs him to find jobs but he doesn't need Mando to get his jobs done, as simple as that.
We don't know that. When we see him use his money it's too quick and out of focus to see if it's the same currency Greef offered him. It could literally be anything and given that he didn't want Imperial credits then, that's the one thing I'm confident it is not.
What's this about consistency?
Yeah, no. Mando did the job so Mando gets paid. Again Mando is surprised to see Imperial credits, that is not their usual arrangement. You can't change the currency agreed upon without consulting the other party and complain when they demand you pay them in something that's still going to be worth something in the near future. Besides Greef offered to give either the flans or the imperial credits to Mando so even if he had a use for them later on, he could use the other half of imperial credits for that. he is clearly willing to part with the flans. And again, again, he could offer to pay the rest later which is what honest people do.
Greef may not be a bank but he is a businessman dealing in semi-legal operations with several currencies, he might as well be one (this is not an attack on bankers, this is a simple observation that his job involves managing large amounts of money in several currencies, like bankers).
So once more, if Imperial credits are stable money why does the dialog states they're not? If Mando is willing to use them to pay people, why couldn't he exchange them to an actual bank to get flans to pay the armourer? And if Mando is usually paid in imperial credits why make a fuss this one time?
I'm proposing that Imperial credits are quickly falling out of value, because as stated by Greef "they still spend" and as countered by Mando "the Empire is gone" and that's why he doesn't want them, that the money he uses in other scenes is not Imperial credits and that Greef scammed him because Greef is shady and Mando is broke enough to work for someone like that as long as still makes a reasonnable amount of money and Greef knows that. There's nothing extraodinary about any of these propositions.
Your proposition as far as I can tell, is that Mando decided he was going to go to the armourer before he had an actul need to upgrade his armor (despite being broke) and so decided not to get paid in imperial credits because the armourer would'nt accept them (despite the fact that he would and that they both have the same opinion on getting imperial-stamped beskar and that he is shown to be extremely dedicated to his Tribe's traditions and not hypocritical in any way) but did not tell Greef beforehand, was surprised when Greef offrered him the currency he ussually pays him with, instisted on something else and took half-payment rather than any other way to convert the full payment into something the armourer would take. Meanwhile, Greef while being completely honest with Mando does not suggest to pay him fully later on and lies by omission about his next job offer. Do I have that right?