From the currencies screen you can enable Tally to handle multi currency invoices and purchases.  Access it by going to Settings > Currencies.  Please note that Tally can currently only raise invoices and purchase receipts in multiple currencies and cannot handle multi-currency payments or bank accounts.  All payments will be recorded under your home currency.  From the currencies screen you can see currencies currently set up in your Tally as well as the current exchange rate (updated automatically every day based on the published rate on XE.com) or exchange rate you’ve manually defined.