It's an "informal translation" per the article so I'm guessing the translator just converted it.
The flyer actually says "50 долларов США" (so, literally, 50 U.S. dollars). And I have no idea exactly why they requested US currency, except that the dollar is considered more stable. Especially in a country undergoing some unrest, the future value of the home currency can be seen as less certain than that of U.S. money.
Also, maybe, some comment on the foreign allegiance of Ukrainian Jews?
Fuckers.
Probably because rubles may not be worth anything soon? The dollar is still more stable. This is nuts. We have family there. Such a crazy situation!