I have invoicing database that contains a portal to items purchased. Since the store does US and Canadian dollars, each item has a price in both currency in the inventory database.

I have the user select which currency for the transaction in the invoice and the lineItems database has a field for the current price, which is auto entered calculated number (price depends if its on sale and its currency) from the inventory DB.

My problem is that, usually on the first portal row, since its a auto enter, the lineitem doesn't have the invoiceID to check for the currency or itemID, so it defaults.

How can a get around this? make a script that creates a new invoice record AND a lineitem record with the needed info entered?

To avoid premature calculation in an auto-enter field, you need to test that all necessary fields contain valid data first. One way would be to enclose your calculation in a case statement:

Case(not IsValid(ParentFile::AnyField),"",

...the rest of your formula here...)