I'm assuming you mean receipt of invoice when you say billed.

When you do this the discrepancy will be posted as a purchasing variance to account specified in product line maintenance.  Essentially it is expensed.

If the item has already been sold then it becomes moot because the costs have all hit the general ledger and it will balance out although they may not be in the exact g/l accounts you wanted.  The bottom line comes out the same.

If you still have inventory on hand and the dollar amount is sufficiently material you can use inventory transaction entry (adjustments) to remove the inventory at the wrong account and then re-enter it at the correct amount.

Example:

Receipt of Good G/L Entry:

Inventory               $1,80

    Purchases Clearing              $1,80

Receipt of Invoice Entry

Purchases Clearing   $1.80

     Purchasing Variance (expense)   $.15

     Accounts Payable                         $1.65

Sale of the Item

A/R                           $3.00

COGS (expense)     $1,80

      Inventory                            $1.65

     Sales                                    $3.00