PO Creation without WBS budget but AVAC activated

Dear Experts,

I have activated budget with AVAC for a project, If budget is available in wbs element i can create PO and post direct cost.

But for some wbs elements there is no budget and the user wants to create PO because initially they will purchase and later they may bill that amount to customer.

But without budget for a wbs element i cannot create a PO here?

Please suggest

Thanks,