What are you going to do about the new Moon Dollars?

Even though my gut reaction is to agree with you, it’s true that it is an app design decision, so in certain scenarios it might very well be a good design decision.

Shipped applications disagree with you.

You never want to be lossy with user inputs. Save what they told you and then convert for internal logic.

The OP just described a language feature where you want to multiply euros with usd one place and yen and dinar in another.

You don’t need to do that which is why a decimal or 64 bit int is a fine language approach.

it depends, if it was a currency exchange, it'll be labeled in one of 3 ways, either average rate per unit, absolute, or it'll be a reference to a MSSQL table containing exceptional spot transactions. Otherwise, a complete history of exchange rates is stored in KDB, most days you can take the average, and for small amounts on weekends or days the exchange is closed you can just interpolate, there's also a half dozen XLSXs full of certain exceptional days, so you'll have to check that you're not on one of those lists. Then if you're not dealing with USD->X or CAD->USD->X things start to get hairy.