Encumbrance in Future Periods

How is it possible to use the Funds from the future periods in the future period ? What is the Oracle Functionality around this.

Scenario is Creating a Requisition for the services that will be delivered over a period of 3 months namely May-10, Jun-10 and Jul-10.

Budget Boundary  Month(Period)

Period                             May-10                             Jun-10                      Jul-10
Budget Avialable              100                                   100                           100

I want to create a Req/PO in May-10 for 300$ of services that will be delivered in May-10, Jun-10 and Jul-10
If I create a Req/PO for total of 300$, it fails funds check since there are only 100$ availbale in May-10.
System does not allow to change the GL Date(used for funds check and encumbrance) to the future periods since those periods are not open.