When creating a Purchase Order for an international supplier, the Purchase Order Currency field becomes available once the supplier is selected.

The currency defaults to the supplier's configured currency.

The exchange rate is automatically populated based on the Purchase Order date. If required, the exchange rate can be manually overridden by entering a different value.

The Purchase Order currency can be changed until the Purchase Order is saved for the first time. Once the Purchase Order has been saved, the currency becomes locked and can no longer be changed.
The exchange rate remains editable after the Purchase Order has been saved and can be updated if required.

Adding lines to FX Purchase Order 

GL line 

GL lines are always entered in foreign currency. By selecting the local currency checkbox, the user can also view the same values in local currency

Pricebook 

Pricebook costs are treated as being in local currency and will be converted to foreign currency using the exchange rate defined on the Purchase Order.

Import Product Line 

When importing a CSV file, the cost value provided in the import is assumed to be in the Purchase Order currency, and no currency conversion is applied during the import process.

Product 

Adding a Product Line will convert the product cost to foreign currency using the exchange rate on PO 

Adding Product with Supplier cost 

For more details on how to add Supplier cost to a product check Multi Currency - Suppliers and Product Supplier Cost 

When a Product with a supplier cost in foreign currency, is added to a Supplier Purchase Order, the most recent Supplier cost will be applied. 

This means the foreign currency rate remains unchanged, and the cost is then converted to local currency using the exchange rate specified on the Purchase Order.

When a Product, with a supplier cost in local currency, is added to a Purchase Order, the most recent supplier cost in local currency will be applied. 

This means the local currency remains unchanged, and the cost is then converted to foreign currency using the exchange rate specified on the Purchase Order.

Please note: Users cannot add the line types listed below when the Purchase Order is displayed in local currency. A warning message will be shown to notify the user.

Add and Receive Purchase Order 

When you click Add and Receive Purchase Order from the Purchase Order module and select a supplier configured for multi-currency, a Currency field will be displayed on the Purchase Order Receipt.
The receipt currency will default to the supplier's currency, but it can be changed if required.
The Exchange Rate field will be automatically populated based on the selected currency and can be updated if necessary.