How to correct or adjust receiving hours of contingent worker after Timcard approval and receivng is

Is it possible to reallocate the amount received on a PO of a contingent worker?  If so, our users need to have the amount received on the PO because some of the hours originally allocated to this contractor are being transferred to another contractor.  The amount billed is far less than what was received and that’s the reason why we need to reallocate the amount received on a PO.