I guess the first question is how do you say financial system. Finance comes from the French finance, which is related to the Latin finis. Still, there is not a native word for it in Latin. For finance, I have found two:
- fiscalis, which refers more directly to public finances, but seems to be used also for the private and general finance sector. For instance, in this Enclyclical by Pope John Paul II, he wrote:
Systemati nummario et fiscali per orbem terrarum
which English translation (and the context of the phrase confirms anyway) is "[to] the world monetary and financial system".
- res nummaria, where res refers to things, and nummaria refers to money. Thus, res nummaria stands for "the monetary things". Is the word res also used in terms of system? Maybe there are some instances that is the case, but I don't know.
Regarding open, liber is an option, if by open you mean free. A free financial system might not exactly be an open one (e.g. it might be free market but only nationally), but in economics, particularly in trade, there is a close correspondence between a free market and a market which is open to trade, i.e. open to the rest of the world.
Thus, I would go for something like "liberum systema nummarium", or "liberum systema fiscale", or "libera res nummaria".
(I hope I'm getting the declensions right. Comments welcomed)