This article explains how Unity handles foreign currencies.
How does Unity convert an invoice submitted in a foreign currency into the currency used in the customer's Unity environment?
Unity supports nearly every currency in the world identified by the ISO 4217 currency standard. Our system uses the Open Exchange Rates API to retrieve currency conversion data and convert currencies nightly.
At what point does Unity lock down the rate so that the amounts do not change (at the time of invoice submission, approval, etc.)?
Once the vendor submits an invoice, Unity stamps the invoice with the most recent nightly rate at the time of submission. The rate does not change after stamping.
Is there somewhere in Unity where the customer can see the exchange rate that was used to convert an invoice?
The exchange rate itself is not displayed. Compare the native-currency amount (e.g., Native Adjusted Total on the matter Invoices tab, or Native Billed Amount in reports) with the system-currency Billed Total to derive the rate.
Is there a way to change the currency conversion rate in Unity to a month-end average rate?
No. Unity uses only nightly rates, as described above.
What restrictions does Unity have on invoices and currency?
Only invoices submitted in US dollars can be adjusted manually or through automated rules.