Before creating the PO reserve the budget

Hi,

Presently once the PO is created with WBS element , SAP is calculating commitments I.e. Budget is reserved for that PO.

Is there any way before creating PO to reserve the budget for that particular PO.

The reason for this is , there are few depts to create PO. For dept A approval came to create POs out side SAP on Dec 1st saying that create POs on Dec 15th. Before Dec 15th Dept B has created and consumed more budget in SAP for their POs. Now when Dept A is tried to create the POs they are getting error that budget has exceeded.

To reserve the budget on Dec 1st it self for Deapt A is there any way . (liked could a fictive PO be done and then be replaced by the real PO)

Thanks&regds

Ramachandra