Purchase Orders for Adagio 8.1B does not allow assigning Event Budgets to Requisitions.

Adagio PurchaseOrders (currently in beta) allows assigning Event Budgets to Requisitions and will give the same warnings or restrictions as occur for purchases. Thus in your scenario, the requisition will be prevented from being posted, and thus the purchase order will be prevented.

We're working hard to get Adagio PurchaseOrders completed as soon as we can, but there is no set date yet for release. There will something for clients on upgrade plans - you won't have to re-purchase PurchaseOrders - but exact details will be revealed when the upgrade is shipping.