Rejected invoices not freeing up the Purchase order amount

Hi,

We are using invoice approval workflow using approval management engine.

Say a PO for 1000 USD is invoiced.

The approver rejected this invoice.

The PO is still showing billed for 1000.

Is this normal ?

Unless we cancel this rejected invoice, we cannot create another invoice as overbilling is not allowed in our system.

Please feel free to point me to the concerned documentation.

Thank you