There is no way to allow for programmatic change of the Exchange Rate in SambaPOS. That is to say, there is no Action we can use to do this natively.
At this time, the only way to accomplish this would be to use some SQL (and maybe JS) that is triggered to update the DB directly, via an Automation Command/Ask Question, or automatically via website query.
@proeftuin even managed to automate this by pulling data from a website daily…