That is a really good question. Your answer is, because it would impose an unreasonable accounting burden on both international banks and U.S. taxpayers (and perhaps impose an unattainable standard), we count the balance (value) of an account as of the close of business each day. We don't concern ourselves with, or need to report, intraday balance spikes and dips.
Thus, in your case, and given the numbers you cite, you would report your overseas account balance as having a maximum value of € 4.200 (4,200 Euros), and not 6,000 Euros
Thank you for asking about this.
