How to prevent saving a rec with variance explanations in multiple currencies?

Is there a way to build a rule that would prevent a user from saving a variance explanation on the primary ledger bucket if they're trying to save it with a different currency than the default currency of the primary bucket? When inputting a variance explanation, users can select any currency from the drop-down, but we only want them to use the default currency of the primary ledger bucket. So if a user changes the currency and tries to save the variance explanation, we'd like a rule to prevent the user from saving it. Currently I'm not seeing an option