Multi-currency in AP

I have a client that is going from a non Multi-currency environment to a Multi-currency environement (FUN, HUH?!)

In the P0411 voucher match, after MC was turned on and set up, we have tried entering a USD transaction with a USD base currency, but the system is still trying to validate the business unit from the purchase order header. They use business units that do not have g/l accounts, so the business unit.gain/loss account (HEB-INV-CG.8870) does not exist.

Can the foreign currency validation somehow be turned off when it is not a foreign transaction?

Thanks, Cheryl