I have discovered a very simple way to do this, such that multiple currencies can be handled simultaneously.
Identify your base currency - SLL for example.
Add 2 custom fields on every customer:
Rate: (Type=Integer; Required; Default value=1)
Currency: (Type=String; Required; Default value=SLL)
For every customer you want to change their currency enter price in you base currency and add the Rate value for alternate currency and the respective currency.
Customer X a EUR client.
Lastly do custom Invoice or Proforma to display calculate value for Totals and display currency.
HOPE THIS HELPS.