You can create billable expenses for your customers. When adding a new expense, select a customer and a new checkbox will appear. Check the billable box and you can convert this expense into an invoice.

By default, the spend amount will be in your base currency. If you choose to make this expense billable, the currency will change to the customer's currency.

Note that if the customer's currency is the system's default currency, the currency will remain the base currency set.