Perfex CRM by default record all transactions (invoices,estimates,expenses,proposals) in the base currency.
Changing the currency while recording a transaction is not possible.
If you need to record the transaction in different currency you need to set customer currency.
Navigate to Customers-> Click on the customer and set currency in the customer profile, note that if the customer have already recorded transactions you wont be able to change the currency. You need to delete all previous transactions to change the currency. Once the transaction is recorded ability to change the customer currency will be disabled.
Try creating new eq. invoice and select the customer the currency will be auto changed based on the currency you set for this customer.
For all future transactions for this customer the customer currency will be auto selected.