Restrict Lead or Opportunity Currency by Business Unit

In our application, different Business Units need to use multiple different currencies when creating Sales Leads or Opportunities.

We use multi-currency, and load daily rates, but when the daily rate is loaded for a currency, it becomes available for all Business Units.

We need to restrict the opportunity currencies to only those applicable to the business unit. For example, the US business unit should be able to choose between USD and EUR, while the Swiss business unit should only be able to see EUR or CFH