If the variation in the Item Receipt rate and Bill rate needs to be shown in the GL accounts only, we can consider the Journal entry approach for the same.
We can enter a Journal transaction for adding the variance amount to the respective GL account directly without impacting the inventory costing method or the Item purchase price.
We can debit the relevant account [common account in IR and Bill] with the variance amount and credit with either an adjustment account or the Finished inventory stock account.
The major drawback of this approach is that the Journal transaction cannot be directly associated with an Item record nor any particular transaction record as NetSuite by standard does not provide the provision of having an Item in Journal lines.
The ability to associate Items to Journal lines is a NetSuite enhancement in the voting stage [#168838]
Another drawback is that with the period in the closed stage, the adjustment Journal entry will need to be made in another period other than the transaction period.