Exchange rate For GST/VAT Tax Reporting

Hello

We have just recently upgraded to One World, and setting up new subsidiaries.

The new sub's base currency is USD. On every invoice that we generate, we need to calculate the equivalent amount in SGD, including the tax amount, and have it printed on the invoice, and to report it to the tax authorities for GST/VAT purposes.

For invoices in SGD, we do not have a problem, because the USD/SGD exchange rate is always captured. But for invoices in USD, the exchange rate is always 1.0, because the base currency is USD, and the invoice currency is also in USD. How can we extract out the SGD exchange rate at the date of the invoice, so that we can calculate the equivalent in SGD, and have it printed on standard PDF templates which we use?