Issue with PO

Hi Experts,

PO# 4550225475  line 10
invocie 81464 (idoc 5193046) for vendor 2000003864 rejected on 7/2/13 for "Quantity invoiced exceeds open PO Quantity".
However,  there is no PO history yet for the PO  (no other invoices and no GR).  There have been no quantity updates on the PO since its issue. 
The PO is open for 500 FT and the vendor ivnoice is for 500 FT.  The PO and line item appear to be fully released and open.
Given the above,  it is not apparent why the invoice was rejected for the Quantity issue.

Please let me know if you figure out any solution.

Thanks

Mallikarjuna