If you are using the AP-C to enter the vendor invoices, any price discrepancy is automatically reconciled so there is no need for them to do anything.  That is the recommended approach.  Enter the vendor invoice and the inventory value will be automatically corrected.  
 
That said, they could also do a negative receipt at the incorrect cost, then correct the PO and do a positive at the correct cost.  Don't know why they are saying they can't.  Or they could change the cost at the time of the receiving if they knew it was incorrect.  There is a default setting to control whether the Receiving people can change PO Price - maybe that is turned off.  SD-C for the setting.